I have not tried this myself because I live in an area where the FCC prohibits the use of an RF jammer. In addition it is even risky to have one in your possession, but since the theory of how this works is educational I’ll post a link to how an RF could be made using the Parallax Propeller Chip. The RF jammer shown in the link below is designed specifically for the Parallax Propeller microprocessor chip, and the Parallax components and parts. The cool thing about using the Parallax Propeller microprocessor chip is that the 8 cogs give you the potential to transmit 8 different frequencies at once off the same unit. Parallax makes a cool little frequency generator.
The instructions for how to make an RF jammer using the Parallax Propeller chip are very simple, they are so easy anyone can do it. But since this is for educational purposes only, do not try it yourself unless you are in an area where it is legal to do so.
RF Jammer instructions from the Parallax Propeller
A design for a self-tuning portable RF jammer
WARNING: In the US and elsewhere the intentional Jamming of RF Signals is ILLEGAL. The FCC does not look kindly on this type of action