I'm sure people can do plenty of harm with the magnetron from a microwave - to themselves! (Lol) Plenty of stories of people getting hurt while messing around inside the guts of a microwave oven.
But I agree that you'd need crazy high power (or better yet, a coherent source like a maser) to have any hope of stopping someone before they ran right past the field. Although here again, the US Military did look into this sort of technology back in the day. The idea was to use high power RF as a "cage" of sorts. The field got stronger as you approached the edges of the containment area, so the theory was that people would naturally stay in the center area where the field strength was minimal.
The problem, of course, is that to be an effective deterrent, it had to be a very powerful RF field (to prevent someone from making a mad dash for freedom and sprinting past the edges of the cage before they were seriously burned). The result was that the "cage" was lethal in certain circumstances, which was the exact opposite of their goal. Also, it would have been very expensive and cumbersome. Not one of uncle Sam's brighter ideas, that one...
Adam