I once got into it with someone on the official forums with someone who complained that forum "banter" was beneath her. Then she spontaneously IMed me in world, and when I answered to explain (politely, I thought) what I had taken issue with in her conduct in the forum, she IMed back "You're a twat. And muted."
I had a good long LOL at someone who would IM me and then mute me for answering.
I think sometimes people expect to be able to get the last word and are shocked to find that the folks at the other end have something to say too.
(Not addressing any of this at person mentioned in the OP, with whom I've not interacted at all - just telling another story about forum-world crossover.)