It doesn't stop it, that's what moderation is for, but it does place an undue burden on rule-abiding users to tiptoe around those who break the rules.
Just telling someone "you're a troll" won't accomplish anything, sure, (though it shouldn't be a punishable offence) but a more useful case might be warning others not to engage with someone who has a history of trolling.
In any case, it's just a thought.