Like HAM radio licenses. The problem is that horse has already bolted out the door. There are millions of RC radio sets out there already. Jamming the link isn't really a good idea. It will generate unpredictable results. Either the drone will go to its loss-of-link instructions or will freak out, either way it will stop doing what it was, which is more dangerous than before if you were trying to avoid it. Pretty much the same problem ultralights have. The only real way I see of solving it is with a (more) practical "sense and avoid" strategy where everything that flies has a low cost, light, low power transmitter attached to them, so that everyone can see each other, and especally UAVs can have onboard code built in to them where they can detect the distance and heading to another aircraft (based on signal lobes) and deviate from their flight path based on it. That might even be able to be built into the existing radio hardware with just a software update. But... that would be too easy, and we are going to get some horribly burdensome monstrosity handed down from on high. Eventually. Probably after some tragedy has greased the wheels with enough blood.