Perhaps the allowed signature styling should change with activity score / membergroup. Like:
- Newbie: No styling (including links) allowed. Max 40 characters.
- Jr. Member: Links allowed. Max 100 characters.
- Member: Unlimited length.
- Full: Color allowed.
- Sr. Member: Size allowed
- Hero: Background color allowed

Then newbies will be less effective advertisers, which would hopefully significantly reduce the incentive for low-content posts. And when people become capable of effectively advertising through their signatures, they'll have invested a lot of time into their accounts, and they won't risk being banned by spamming.