We all know what hellbans are: it's when a poster is the only one who can see their own posts, but is not informed of their invisibility to everyone else. Diabolically effective at dealing with certain types of troublesome user.
Sometimes, however, I wonder if it would be a neat trick to do something similar, but only for specific pairs of users.
In other words, you'd make it so that person A cannot see person B's posts -- and maybe vice versa. And maybe voluntarily: it could be a form of highly selective hellbanning, to reduce tension between two otherwise productive members of a community.
Or it could be something you set up yourself: "Hellban me from being seen by person B"
No, I'm not naming any names!