Forum Signatures and Input Format Filter

Hello. I have a little problem with signatures in forums.
I use Advanced Forum. Users can add a signature in its own profiles that is shown on the forum post.
That works good, but the problem is some users has access to change the input format filter in this signature, and then, they have change it to a FULL HTML and enter their custom signatures.
The problem is that users who hasn't access to change input format filter, cannot see the signatures of this users right.
It's very rare, because anonymous/authenticated users can see node pages with input format filter in FULL HTML, but they can't see the signatures of forum post.
How could I fix it?