How can I protect components from being stolen off my bike? I had my bike locked up properly so that the frame and wheels were secure, but somebody stole the 8 speed and handlebars off of my bike. I don't understand how this can have more than a $10 resale value.

The bike is a Shimano 8-speed. They cut the tension cord in 2 spots... i'm not sure why they cut it near the derailer.... 

Is there any way to protect the gears? It looks like the other one that remains is susceptible to just an allen wrench and wire cutters...
There's no way to 100% secure everything, but there are things you can do to make it more difficult or time-consuming.


Don't lock up your bike. Bring your bike with you instead of leaving it outside. Mine goes into my office.
Choose a location where you or other people will see thieves messing with the bike. Not a super-high traffic area (in that case everybody assumes somebody else will do something), but a moderate traffic area. A thief will be more reluctant to do something obviously shady if there's people that might stop them. When I'm at a cafe, pub or restaurant, I try to lock up where I can see the bike from inside. Or use bike lockers of some sort where it's hard for thieves to get to any of the parts.
Use locking skewers such as pitlock or pinhead. There are a lot of variations on the concept, but the ones I've seen can secure wheels (via skewers), seat post (seatpost collar), fork (headset cap) or some brakes. A locking skewer would protect the casette from being removed, but not the rear derailer.
Replace the bolts with unusual bolts. Bike thieves are likely have a variety of sizes of allen wrenches (possibly in a single multitool) and maybe a T25 Torx (commonly used for disc-brakes). Also Phillips and flat-head. But they're unlikely to have other sizes of Torx or other more unusual types of bolts. Getting proper "tamper-resistant" bolts is difficult unless it's something like the pitlock stuff.
Mess with the bolts so that they're harder to undo.  One technique I've heard of is to put a ball bearing into the hex head and melt some wax/paraffin into it. Tilt the bolt, heat it with a small flame and the wax runs out. Or a bit of time prying and scraping with a small screwdriver.


Personally, I do #1 and #2, and have considered investing in #3.  #3, #4 and #5 all make your bike harder to work on, but at least #3 bike shops are used to and you just make sure to bring the special tool with you all the time.