Steal a tacocopter and stick a granade under it -- that should enable you to take out the target (and leave a pretty decent mystery for the local authorities to solve. How do you prove who flew the plane? Who fixed the handgranade?).
I don't think it's that simple. For starters, there's probably a very short list of people who own tacocopters, let alone people who own grenades.
If it becomes a problem, I can think of countermeasures -- register the things, ban them in public spaces/require flight plans, detect and triangulate the control signal. Heck, you could just look around; with a low tech system like that, whoever's flying the thing needs line of sight to it in order to identify the target. It's noisy and slow and I'd expect not nearly as effective as stealing a car and doing a drive-by shooting.
But more than that, I'd be very surprised if that sort of thing becomes a problem. Normal people aren't murderers. Think about all the poisons, explosives, and highly kinetic pieces of equipment normal people have access to right now.
Getting a grenade is pretty easy, and I meant stealing the tacopter to get around the flightplan issue you can order him the taco using a stolen credit card, the intercept it using a butterfly net.
