I think it’s possible in theory, but I’m not finding an easy way to do it right now. Right now what I’m doing is using decorateCooked, which doesn’t seem to allow easy access to the parent element where the reply information is. I might fiddle with this more and see if I can get it working, but no promises.
This will block mentions of a username in the post if the username is included in the block list; if whole-word matching is on, the username in the blocklist needs to include the @mention, but if whole word matching is off then only the username without @ can be included.