[abeD]

You don't block all frequencies but one (if you could do that simply then you would be making alot of money) but with some combination of low pass and high pass filters you can get a nice bandpass of filters centered around where you are transmitting. Do a wikipedia search on filters and try to get through it. All of the stuff I've learned about is college level engineering so it might not be the easiest thing to understand.

There are also several types of active filters as well that have a sharper "edge" to them on their bode plot. (a magnitude vs. frequency plot).