That's an issue that would require a fundamental redesign of how our ignore system works.
1) Ignore Public posts by user x. 2) Ignore Public replies to user x. 3) Ignore (block) PMs from user x.
Plus, a global toggle of #1 and 2.
Personally, I'd like to see us go (back?) to that, but it's not a high priority at the moment. I know we used to have that level of control on SI, though some of it was lost in the '03 rewrite.
"... the ability to ignore messages between two users. Sometimes people who generally make worthwhile posts get so upset at each other that their posts to an from each others are just venting and flames."