There has to be a way to delete them from user A's end without removing them from user B's. If I know I'm not going to participate in a PM thread, I should have the option to delete it from my end so my count doesn't keep skyrocketing and getting my hopes up.

I guess a better term would be dis-involve myself from them? Just as long as I don't have to look at the ones I don't want to see anymore, nor am getting their message pings.