Stopping myself from replying immediately has saved my bacon more than a few times.
Often, I'd realize that I don't want to send it at all. The emotion was processed and now it's fine.
Sometimes, there is a valid point (e.g. because someone was rude in tone) and I'd just give that as feedback in a non-violent manner.
Most of the time if someone is being intentionally rude on the internet, they're also being a troll. Don't feed the animals!