Sometimes we do, if they behave themselves, and sometimes we reinstate somebody if he asks nicely (and watch him, to make sure he behaves nicely too). There are a few who will never be allowed back in, though. When we say "one last chance" we mean one last chance (the trolls don't mean what they say; we do). Some people screw up so badly the first time we won't give them another chance, too.
Were you thinking of someone in particular?