Sonic stunners (currently in use aboard ocean liners to repel pirates)?
Dazzlers (high-intensity flashing lights that disorient & induce paralysis, used by some current hostage-rescue units)?
Hmmm.....
Neither of those are canonical. (Which means they have to be in CT, MT, TNE, T4, or MGT... T20 is only setting authoritative...)
Even then, it would be extremely bulky, and require large amounts of power.
There are a couple of good explanations of how a stunner could work (Larry
Niven's ultrasonic stunner, for instance) but power once again must be
considered.
Hmm, sonic stunners might be better than gas dispensers for the 'shipboard security' thread.