Robert Holmes villainidea - when you block someone, you can type an optional reason, visible only to you in your Blocked Users page. this way, you can easily remember malevolents ('called me a slur' 'climate change denier') and sort them from good but temporarily annoying people ('not interested in their Discourse thread on day when I had bad family news')
LiveJournal paid accounts let you write notes about people's journals which would display if you hovered over their userpic. People would use them if they were banning people from their communities to keep a note of how bad they were (or weren't).