Certainly in such cases, both users should be blocked.
I am particularly disappointed with Anthony's statement "(fine, if he can violate the 3RR then so can I)". This is in very poor taste, and that kind of retaliatory behavior is what we should all strive to avoid whenever we can.
Take the higher path, improve your own behavior, even if others do not. Never sink to the level of an interlocutor who is behaving poorly.
This principle servers multiple purposes. First, it is good for you, it is something you can take pride in. Second, it helps the community to sort out who the problem user is... leave a clean paper trail. Third, it can very often inspire the counterparty to pay you the same courtesy in return.
--Jimbo