So what is a Jammer?
Def: jam 7. 'To interfere with or prevent the clear reception of (broadcast signals) by electronic means.' - http://www.answers.com/jam
This instructable show you how to build a simple RF jammer/noise generator for 0-128MHZ. The sole purpose of this device is to interfere with the proper function of radio controlled objects.
The best part is that this project is SIMPLE. All of the heavy lifting is done for you by an Parallax Propeller chip. The hardest part is waiting for your components to arrive by UPS.
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.
Also lady ada has a great writeup on this: