While that is a glorious achievement of this Chaos Brigade, does it matter that they changed the stamps?
Maybe I'm assessing the situation wrong, but I imagine your campus is rather big and there are designated good spots for postering all over the place. To distinguish a scanned stamp from the real thing is going to take examination from no more than 1 or 2 ft distance. Is some janitor / facility management person really going to check all the poster spots from up close to determine whether the stamps are real? Really?
I say this Chaos Brigade should not stop before the Mgt decides it is necessary to use specifically UV waterproofed paper for posters to determine authenticity.

No but seriously, the only thing I can imagine is that this Mgt may spot one fake stamp on a certain location and can then continue to quickly remove all posters that look exactly the same without close inspection. In which case, you should most definitely abuse the fuck out of that piece of security theater by colour scanning + printing some posters of boring shit you don't like and make their reproduced stamps look just a bit more fake than they should, and hopefully prompting the Mgt into carelessly removing all of the boring posters even if they have authentic stamps.
Do campus organisations also get sanctioned if they put up unstamped posters? Because, you know, more fun can be had.