If I am not mistaken, I recall a similar incident about mp5s as well, but the community decided that mp5 is much more advanced mindpower level to 'ask for permission' thus the rules were only for mp3 and mp4.
I haven't seen yet any mp5 saying that the SG is only a suggestion and who ever wants it, follows it or not. But if that is the case, then I am disappointed. Don't forget that we were mp3 and mp4 once, the older players should remember how we needed a place to train without being attacked randomly by everyone.
If you want to avoid future complaints as this, then I would suggest having people there who reinforce the rules. Yes, staff is what I mean. A person alone can't do evertything. A signpost isn't enough.
Also a retaliation, like Nadrolski suggested, sounds like the best thing for me.