Just restrict signatures to members and above.
Or restrict lower ranked signatures to just one line of text - no formatting. That is a traditional signature.
I dont think thats a good idea because when the merit system was implemented, juniors werent able to have clickable signatures, but they still had non-clickable ones, and many spamties campaigns were paying them to post crap with those signatures.
I agree with hilariousetc that signatures must be restricted to members and above.