Most but not all GPS trackers can be blocked! However the police are now actively looking for the Jammers and they are easy to detect. Due to the fact that a lot of things now use the GPS timing signal the police tend to through the book at you for doing this, this can include counter terror laws..... the http://www.tracker.co.uk people do one called the Horizon this apparently uses multiple whays of sending out an "i've been stolen" shout and multiple ways of locating it!

what ever you do at least make their eyes water if they try and take your pride and glory!