There are jammers for sell outside of the U.S. Do not get caught with one. These are highly illegal!
You can get in trouble in many different ways. Someone has paid for the rights to use these freqs (such as cell phone providers etc.) and you can be prosecuted for theft of service.
Most "jammers" on the market kill a wide spectrum of freqs. Fire one up in a downtown business district and you will kill wireless access, cell phones, pagers, comercial phone wireless headphones, wireless cameras, as well as other devices.
If you fear an "Attack" you can try to overcome this by using higher wattages rather then using a wattage that is just bearly getting your signal to it end point. Using poles, or towers or mounting antennas on the highest part of the building can help get you above "street" level attacks.
If you plan on setting up a lot of wireless projects you can get info from the FCC on all of the listed transmitters in you local area. Everyone is aware of radio stations, tv stations, and other transmitters but sometimes people are not aware that commercial accounts have transmitters such as plumbers, electricians, school bus districts, and no one thinks about all of the fast food drive thrus. When you see one of these freq lists you will be very suprised how many listed transmitters there are within a 5 mile radius!
If you attack one of my systems I have radio direction finding and can triangle you in less than 5 seconds. Video is emailed to all law enforcement agencies involved and they are in turn emailed to the responding squad car. I hope you can run very fast! Dont forget to bring a disguies change!