I think having the ability to delete every post by a user would do more harm than good. Unless trolls have been banned by the Council, any sensible posts they make won't be deleted (that's providing they do actually make a sensible post). If we deleted all posts by a user, it would wipe out any potential non-troll posts they have and with them all the replies to those posts by other members.
Mod deletion already is pretty easy though - the mod panel has a section showing posts in reverse chronological order. If a troll goes on a rampage, all we have to do is delete all their posts from that page - it's pretty simple already! We can delete many posts in the time it takes a troll to post once. It's not like we have to go through the forums hunting down each individual post by the user!