Showing off your bans.
It's said clearly in the BBS rules:
Complaining or boasting about any ban, including comments in signature text.
This rule applies to BBS signatures as well.
The reason for this is that showing off your ban is doing two things -
1. Showing that you obviously didn't learn your lesson from your ban since you're treating it as a trophy of your recent rule-breaking.
2. Encouraging other users to misbehave so they can eventually be banned and have their own ban signature to show off.
Being banned from the BBS isn't meant to be a good thing. It's not supposed to be another excuse to show off.
It's supposed to be a private, user-specific notification that you broke the rules or overstepped your boundaries.
If people continue to put their bans in their signatures, the bans themselves will have less and less impact because they'll be a common, trivial sight of everyday BBS browsing.
BBS moderators can delete your Signature text and picture, so uploading and replacing something that was deleted will be seen as an act of defiance to this rule, and might get you banned.
If you notice something missing one day, try PMing a mod with a link to your missing signature image and ask them if it was breaking any rules.