In "Forced Perspective", there's a man who has a big bomb in a car. It can be triggered by a radio signal. Peter blocks the bomb from getting triggered by having everyone turn their radios to 432.500 MHz. Is this something that would work in "real" life, or is it just a sci-fi made up trick?
It depends: if those radios are emitters as well as receivers (i.e. walkie talkies) AND you could get them to emit a stronger signal than the trigger, all at the same time, it might just work (see Chris' answer for more details). According to Wikipedia, 432.500 MHz is used for amateur radio in the USA, so this sounds like a plausible scenario.
But for regular radios like car radios, it would never work in real life.
It is true that any antenna that receives a signal weakens the electromagnetic field that constitutes the signal, but only by a very small amount, and only in the direction "behind" the antenna.
So theoretically, you could build a "wall" of antennas around a receiver that blocks it from receiving any signal - this is called a Faraday cage. But a bunch of car radios isn't going to have that effect.
Again theoretically, it is possible that the signal is so weak that without any disturbance the receiver would just barely detect is so that the tiny weakening from nearby antennas would be enough to make a difference, but this is extremely unlikely, literally a one in a million chance even if the signal were deliberately kept weak.
Finally, it would not matter at all what frequency the radios are turned to, since their antennas receive a broad range of frequencies and "turning to" a frequency just means other frequencies are filtered out before the signal is amplified.
Oh, and all of the above is a gross simplification of all the effects that play a part in radio transmission and reception.
There are a couple of (reasonable) assumptions tied up in this:
It is a SPECIFIC signal on that frequency that the bomb needs to detonate, not just any signal of a sufficient amplitude on that frequency. Like I said, this is reasonable, because you wouldn't want it going off by accident.
Given the distance and relative power of the radios' transmitters, their signal is sufficient to block the signal. Whenever you hear in the movies "JAM THE FREQUENCY!!!" this is what they're talking about...they're simply flooding the area with a signal at that frequency sufficiently strong to swamp the target signal.
The reason why bomb squads don't try this all the time is that generally, you have no idea what the frequency is...even so, you can try setting up a Faraday cage around it (which, to varying degrees of effectiveness, block ALL EM signals). Here's a bomb squad strategy organized around both of these concepts together:
All in all, this was by far one of the more reasonable things you see on Fringe :).