Uh, super. Are you missing the fact that if these are flying over your city they are paid for by your taxes and the police will just buy another one, probably a bigger and more expensive one, justifying it by saying that "criminals are shooting them down because they're scared of them" or something?

Also, if you think that a trip to a hackerspace qualifies you to build a UAV, I really don't want your armed UAV flying over my head. Maybe start with a $1M+ budget and a team of qualified aerospace, mechanical, and electrical engineers. Ones currently with well-paying, legal jobs willing to leave those to work on your angsty teenage temper tantrum that would ultimately be counterproductive anyway. I have experience with model airplanes and robotics and I'm against this as much as the next guy but if you asked me to join your team I would report you to the FBI before a model airplane with a shotgun duct taped to it crashes into my roof.

If you really want to take criminal action to stop this, you should... no, I'm not going to tell you, lest you're dumb enough to try.