Most of these comebacks are absolutely terrible. Here's a few that I use regularly. No you shut up, you'll never be the man your mother is!
Why don't you slip into something more comfortable, like a coma.
I would say something intelligent, yet cutting to the soul.
'Firstly, you have no way of making me shut up. Secondly, if I obeyed the command of every idiotic troll on the internet, you wouldn't be here talking to me now. Yet another idiot that talks before they think, that opens their trap and comes up with yet another reason why people are stupid...'