Can I force my MacBook to only charge from the USB-C device I select? Or forbid it from charging from devices I know to be unreliable? For some reason, my MacBook keeps trying to charge off of my external display's USB Type-C ("USB-C") link even though I have a more powerful source connected. 

I care because the monitor is apparently unable to provide the 45W it reports, and after a few seconds the system switches to battery power, and then immediately back to AC power, over and over. This has the effect of flashing the screen on and off, and causing mouse / keyboard interactions with my remaining screens to momentarily pause. Overall, it makes the setup useless.

Here is what pmset -g ac has to say about the monitor when it is the only device connected:

# pmset -g ac
 Wattage = 45W
 Current = 2250mA
 Voltage = 20000mV


and the same for when my power source is the only one connected:

# pmset -g ac
 Wattage = 60W
 Current = 3000mA
 Voltage = 20000mV


When they are both connected, it reports 45W as though the monitor is the only device.

Log showing the endless loop. Initially I am truly on battery, and then I attach the monitor which triggers the first Now drawing from 'AC Power', everything else happens by itself after that:

# pmset -g pslog
Logging IORegisterForSystemPower sleep/wake messages
pmset is in logging mode now. Hit ctrl-c to exit.
2018-03-06 14:45:01 +0000 IOPSNotificationCreateRunLoopSource
Now drawing from 'Battery Power'
 -InternalBattery-0 (id=4128867)    94%; discharging; 6:10 remaining present: true
2018-03-06 14:45:08 +0000 com.apple.system.powersources.timeremaining
2018-03-06 14:45:08 +0000 IOPSNotificationCreateRunLoopSource
Now drawing from 'AC Power'
 -InternalBattery-0 (id=4128867)    93%; AC attached; not charging present: true
2018-03-06 14:45:08 +0000 com.apple.system.powersources.percent
2018-03-06 14:45:08 +0000 com.apple.system.powersources.source
2018-03-06 14:45:08 +0000 com.apple.system.powersources
2018-03-06 14:45:08 +0000 com.apple.system.powersources
2018-03-06 14:45:09 +0000 com.apple.system.powersources
2018-03-06 14:45:10 +0000 com.apple.system.powersources
2018-03-06 14:45:25 +0000 com.apple.system.powersources.timeremaining
2018-03-06 14:45:25 +0000 IOPSNotificationCreateRunLoopSource
 -InternalBattery-0 (id=4128867)    93%; charging; (no estimate) present: true
2018-03-06 14:45:25 +0000 com.apple.system.powersources
2018-03-06 14:45:31 +0000 com.apple.system.powersources.timeremaining
2018-03-06 14:45:31 +0000 IOPSNotificationCreateRunLoopSource
Now drawing from 'Battery Power'
 -InternalBattery-0 (id=4128867)    93%; charging; 0:00 remaining present: true
2018-03-06 14:45:31 +0000 com.apple.system.powersources.percent
2018-03-06 14:45:31 +0000 com.apple.system.powersources.source
2018-03-06 14:45:31 +0000 com.apple.system.powersources
2018-03-06 14:45:32 +0000 com.apple.system.powersources
2018-03-06 14:45:33 +0000 com.apple.system.powersources.timeremaining
2018-03-06 14:45:33 +0000 IOPSNotificationCreateRunLoopSource
 -InternalBattery-0 (id=4128867)    93%; discharging; (no estimate) present: true
2018-03-06 14:45:33 +0000 com.apple.system.powersources
2018-03-06 14:45:33 +0000 com.apple.system.powersources.timeremaining
2018-03-06 14:45:33 +0000 IOPSNotificationCreateRunLoopSource
Now drawing from 'AC Power'
 -InternalBattery-0 (id=4128867)    93%; AC attached; not charging present: true
2018-03-06 14:45:33 +0000 com.apple.system.powersources.percent
2018-03-06 14:45:33 +0000 com.apple.system.powersources.source
2018-03-06 14:45:33 +0000 com.apple.system.powersources
2018-03-06 14:45:33 +0000 com.apple.system.powersources
2018-03-06 14:45:33 +0000 com.apple.system.powersources
2018-03-06 14:45:34 +0000 com.apple.system.powersources
2018-03-06 14:45:34 +0000 com.apple.system.powersources.timeremaining
2018-03-06 14:45:34 +0000 IOPSNotificationCreateRunLoopSource


So: Can I force my MacBook to only charge from the device I select? Or forbid it from charging from devices I know to be unreliable?

System: MacBook Pro (15-inch, 2017) with 3.1 GHz Intel Core i7 and 16 GB RAM
Has this issue been resolved?  I have a couple ideas on this, perhaps not useful for the original poster but maybe for someone that sees this same behavior.
First thing though is that this is very odd behavior.  Apple documentation shows that the MacBook should take power from the larger power supply regardless of which is plugged in first.  This behavior may show that there is some deeper hardware or software problem.  The need to track this down is subjective.  The need to get back to work is likely a priority and so resolving this is likely the highest concern for anyone in this position.
The quickest way to resolve this is to simply remove the power connection from one supply or the other.  This can mean just not connecting the 60 watt power supply and running off the 45 watt supply from the monitor.  The 60 watt supply was chosen for inclusion with the MacBook in order to allow for charging the battery even when under high CPU load.  Normal use is rarely going to need all this power, especially if you are using this MacBook while it sits on a desk for hours at time.  The best part about this solution is it costs nothing to implement.  The bad part is it does nothing to determine if there is a deeper issue causing this behavior, as well as does nothing to resolve it.  If this gets you back to work then that may be all that concerns you, and this may be all that needs to be done until the computer is retired.  Or, the display is retired.  Or, you are retired.
Breaking the power connection from the monitor can be done with a different cable.  It sounds like the monitor is connected with a common USB-C to USB-C cable.  Presumably the monitor has a DisplayPort and/or mini-DisplayPort input to supply DisplayPort video in addition to the USB-C port.  An adapter cable from USB-C to DisplayPort will keep the DisplayPort video connection but break the power connection.  If there is only USB-C for DisplayPort input then a USB-C to DisplayPort adapter with a DisplayPort to USB-C cable will also break the power connection and keep the video connection.
Another option which might be attractive because it can solve other problems along with this odd behavior is to get a USB-C dock.  The dock will connect to the MacBook by a single USB-C port and split out the video, power, and USB.  This means the MacBook will only take power from your power brick, the video will be split out to a DisplayPort port, and then USB for mouse, keyboard, and whatever else.  The connection to the display can then be by a much less expensive DisplayPort cable, assuming it has a DisplayPort port in addition to the USB-C port.  If there is no DisplayPort input then a DisplayPort to USB-C cable should be able to connect the monitor and still break the power supply connection to the MacBook.
Try running the MacBook on the display power, it costs nothing to try.  If that leaves you with the battery running low too often for your comfort then look into the other two options.  Which of the two works for you at the lowest cost will depend on the ports available on the display and what else needs to be connected to the computer.
I found this problem interesting, I would like to know how it was resolved.