When talking about a poster who has already been banned for misbehaviour, is it acceptable to call that person a troll?
It’s a tad unfair to take potshots at someone who can’t come in and defend themselves, so I don’t think so.
Actually we’d prefer once someone has been banned not to speak of them again. They were such that we had to remove them from the board; what’s left to talk about?
Of course we’re not talking about the Pit, where “troll” is the least you can call someone. But that is an exception. 
If I may expand a little on what TubaDiva said: the whole point of trolling is to get attention. Negative attention, that’s fine, as long as it’s attention. Thus, making reference to banned trolls in any way is giving them what they want. In a phrase, “the trolls win!” Seriously.
The best response is to give them NO attention, NO recognition. Don’t stroke them at all. They get tired and go away.