Simply as the title says. Restrict any weapons on a mech (immolator / mg / whatever) to only being useable (or maybe even printable) on red alert. Use an emag on it to disable this restriction.
Far too many antag runs have been stopped abruptly, sometimes with little RP on either side, just because big stomper moment. Besides, the only reasons crew should be using mechs is if it's a scenario that also warrants red alert - you know, like antags using mechs (among various other things). One-sided combat like this just isn't fair, or conducive to good RP.
Probably some stuff I've missed or not considered, if that's so, post a comment and I'll respond when I get the chance - I might even change my opinion if you present decent arguments.
My bad if this has been posted before - I looked, and couldn't see it.