Well, even during his prime, Bazooka had a nasty tendency to his opponents when they were down. That's apparent against Zarate, and against a few other fighters as well.
Riddick Bowe could be dirty but rarely lost a point... others... Hopkins Trinidad Ruiz Benn Hide Duran
Ali was notorious for holding behind the head and didn't get warned enough about it. He also put his forearm on Foreman's throat in the first round. I love him but man was that guy dirty.
I was gonna say Conteh. He could be quite handy with the head. Chris Finnegan called him "old onion head". He was convinced that John must have soaked his head in vinegar to toughen it up. Zivic was the first that came to mind for me.