The same thing that stops people driving around with GPS and cell-phone jammers. That is, nothing, apart from the threat of prosecution if caught. This seems t

The same thing that stops people driving around with GPS and cell-phone jammers. That is, nothing, apart from the threat of prosecution if caught. This seems to be a common complaint about new technologies; people claim that somehow anti-social, violent or abnormal behaviour will be a problem merely because it is _possible_.