Can you block the GPS tracking device?

Yes, you can. You can buy cheap blocking device - so called GPS Jammer online from China for a price of good dinner - around €30.00. Usage of it is prohibited in US and Europe, but  no one can stop Asian manufacturers for selling them. Your position is calculated by triangulation method and GPS is a radio frequency signal ( by the way, very weak signal, as the satellites in the space don't have vast amount of energy to make it stronger). GPS Jammer just blocks that frequency, which means any GPS device in your car and around it (range is about 10 m) would stop receiving GPS signal from the satellites - so your car navigation, GPS enabled mobile phone and GPS tracker would become useless.
GPS jammer mostly would be used by car thieves, but sometimes company car drivers might try to use it for off-route lunch or friends' visiting. In this case our system shows GSM signal, but no GPS, so it's still possible to track vehicle and driver cannot get away with statements about faulty device.
Though there's another sytem, which might replace GPS technology- it's called ELoran. The advantage of this system is that it gets positioning information from the ground sources, not from satellites, thus making that signal much stronger and very difficult to block or jam.  But USA rejected the development of ELoran, staying with GPS system, and Europe is still in doubt, although scientists say that would be easy adapt existing GPS system to work as ELoran standard.
Also don't forget GLONASS - global positioning system, operated for the Russian government by the Russian Aerospace Defence Forces. GLONASS is currently the most expensive program of the Russian Federal Space Agency, consuming a third of its budget in 2010. By 2010, GLONASS had achieved 100% coverage of Russia's territory and in October 2011, the full orbital constellation of 24 satellites was restored, enabling full global coverage.
So I guess we just have to wait a little bit to see who will offer most reliable and affordable solution to work worldwide.