The biggest deterrent to prevent hoons, nimrods and nupties from buying these things is to price them upwards
The cheap ones won't get very far, being quite limited in radio range. Not to mention if they get anywhere near high enough, the nupties, nimrod or hoon will probably lose control of it anyway due to their lack of being able keep it level from a distance which is quite a skill...
The scary ones are the weekend warriors that put their own r/c drones/planes together. Especially with FPV now. These things are more than capable of flying 6000-7000 ft and quite some distance using a UHF long range link.
It's cost me so far about $500 US to put together a FPV electric glider. I've just finished soldering together the avionics and nav system. Damn thing also comes with a autopilot and auto leveler and will fly itself home if it loses the radio control signal. I haven't dared to test flight it for max height yet, but I wouldn't be surprised if it topped 8000-10000 ft, esp with the more powerful transmitter that was fitted...
Its a hell of a lot easier to crash too that a real plane, as flying though video goggles is fairly different to just jumping in a plane... I've already put one down in the sticks about 2 km away... and that was sub 500 ft alt..
TCAS won't pick up a drone. I don't see your average person buying a $5000 transponder for a model airplane! The trick I believe is good education and line of sight flying only.
Its the uneducated folk I'd be worried about.. the ones with no idea of the airspace around them...