At some point, they will realize that this is happening, and there will be outrage when they communicate about it to their friends via an outside channel. Be prepared for a lot of splashback.
I've thought about this, but it basically means I'm programming features for assholes. I don't want to get in a war like that, but I may have to, I guess.
HN is full of programming features for assholes, it seems to have served the community well, even if it isn't always 100% transparent and in the case of false positives it requires manual undoing.
Hmm. Has anyone tried a purgatory ban (I just made that up). Basically give individual users the ability to never see posts or messages from other individual users, but no indication to the ban-ee.