How do I block power from going to usb without stopping data transfer? I need an iPad connected to my iMac for a long time, but I don't want power going to it because leaving an iPad charged to 100% all the time ruins the life of the battery(link). I still want to allow data transfer, but with no charge(or extremely little like when it says "Not Charging") going to my device.
There is no easy way to do this. You could try to rewire a USB cable with the power wire removed. There's a chance this would work, but I wouldn't bet on it, it would depend on how the iPad handles USB connections. 

You won't ruin your battery by having it plugged in at 100%. This might be an issue if you're leaving it 24 hours a day, but Apple is smart enough not to let their batteries die from being plugged in.