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.
Oh, I'm holding out for requiring RFID tags on posters. What gets me is that they apparently only enforce the rules if they feel like it. There are flyers advertising
student employment pyramid schemes all over the place, completely unstamped, that have been up for years in some cases - because it's public space and that means it's the job of someone else to take care of cleaning things up.
In our case, it wasn't that the stamps were detectably fake (except for the ones dated 2012 and such), but that people took the "obscene" posters to complain the the lady in charge - "Why would you ever approve this?!? It says 'Curiosity killed the cat; don't be a pussy' on it in big letters! PUSSY! That word is so offensive and obscene and sexist and I AM NOT A PRUDE BUT I'M JUST SAYING... " blah blah blah. Apparently, the people in charge of approving the flyers caught a lot of flak from people who believed the flyers had been genuinely approved. We got in trouble not because we broke the stated rules, but because we annoyed people in charge. Marianist Values®!
Our MO last year was to print off one copy of each flyer and then B&W photocopy them at $.05 each - 5 bucks is enough to put multiples in every stairwell on campus. I guess they're thinking that we would need a color copy at $.75 each, and wouldn't make any because it would be prohibitively expensive? But this year, I have my own quality color printer available to the Brigade, so it just means we'll be making brightly colored eye-catching posters since we have to use color anyway.
