What prevents someone from hijacking a radio frequency mid-air? For someone who wants to stay anonymous and probably terrorize the skies, what protective measures do we have?

Without even thinking too deep, I can imaging the following scenarios for someone who's in a mood to mess around.


Flying in a busy airspace, calling to "Cancel IFR" for someone else.
Abusing recklessly. Maybe stepping-on the frequencies infinitely. 
Reporting fake traffic or fake weather conditions, causing re-routes
and delays.


While transponders are required in and around busy airspace, someone who really sets their target on screwing it up can REALLY cause a lot of harm, potentially fatal by flying without one for quite some time as a primary target on ATC's radar.

What are the current preparations to stop this from happening?
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.