What prevents someone from hijacking a radio frequency mid-air?
This sounds like a “people problem”. That kind of problems is best solved with enforcement.

Source of radio broadcast can be localized and it is not particularly difficult. There are techniques for transmitting something secretly, but they need specific modulation. Rogue broadcast or jamming on simple AM voice channel will be trivial to triangulate.

So if somebody did it, the FCC or FBI would pull out appropriate equipment, hunt them down and throw them in jail. Broadcasting without license, or contrary to the license you have, is in itself a crime and they'd throw in charges for endangering, which are quite serious.

Now of course it would take some time to hunt the perpetrator down. However, the situation is not that dangerous. Basically it could be treated as communication failure. VFR flights would continue flying visually, IFR ones would follow last clearance and then as filed, landings can be cleared with a light gun. And then either the surrounding sectors would be OK, or the pilots would probably try to establish some alternate communication. Cell phones should work fine for small aircraft while airliners would probably use HF or Sat links to dispatch and ask the dispatcher to relay.