Although the GPS signals are very weak they are very resistant to jamming. In fact all of the satellites transmit on the same frequency without interfering with each other. The technique used is know as spread-spectrum coding. Each of the satellite
signals are bi-phase modulated using one of 64 "Gold" codes. These codes are unique in that they do not interfere with each other. Additionally, the navigational information transmitted by the satellite is formatted in a way that it must match information transmitted by the other satellites.
The GPS receiver must match the code used by the satellite in order to receive the its signal. This matching process make the satellites signal stronger and any interfering signal weaker. Further, if the received signal is not correctly formatted and does not have the correct navigational information is will be ignored. It also takes a minimum of 3 satellites before the receiver will even try to compute a fix.
These characteristics make it nearly impossible to spoof or jam the GPS system. I am aware of only two ways to disable the system short of blowing something up. These involve the use of a signal on the GPS frequency that is so strong that it desensitize the receiver, or the transmission
of a very strong random noise
signal that buries the GPS signal. Both of these methods are very local and targeted. They are not useful over a wide area. Also, receiver will detect these forms of jamming and warn the operator.