It can be codes and text. I think the main roles are:
Newbie - 24 characters
Jr members - links
Members - size
Full members - avatars
SR members - can use up to 4000 characters (I think) and colour.
Hero/legendary - can use bold fonts.
Each inherits previous rank signature allowances.
- 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.