The only other suggestion I can think of is something I've suggested before: only allow signature campaigns that pay in Bitcoin. The ones paying in made-up tokens have no real cost for the ICO, and thus don't mind "paying" for spam. If the campaign pays in Bitcoin, at least they have something to lose.
I totally agree with this suggestion. The signature campaign is a significate marketing strategy of the forum so removing it will remove the vibrant color of the forum engendered by the various signatures. But keying to this suggestion will mean that any signature campaign will be a serious one. it will also reduce the spate of scam campaigns since it will be easy to identify campaigns that are serious from unserious ones.