Is there a documented process in 'Jamming' an NXT Ultrasonic sensor?
Since everything is fair in love and war, you could do this:

Using an Arduino and this NXT compatible shield, you could program it to send a continuous burst at 11494,253Hz creating noise in the frequency range of the NXT Ultrasonic sensor.



This site shows how to use the Shield with the sensor.  All you have to do is modify the code to create a long burst instead of a controlled ping.