MNHQ have just deleted a thread (the one about a violin exam) for this reason. However, this was in practice down to one poster alone who clearly came on with the deliberate intention of being as thoroughly unpleasant as she could. The thread could have been put back on track simply by deleting her posts and probably banning her until she sobered up calmed down.
I do think at times MN is too quick to go for the easy option in cases like this.