Looks like the new version of phpBB has a feature along these lines called “Friends and Foes”. If you mark someone as a friend, all their posts are specially marked and appear in a special Friends section. If you mark someone as a foe, their posts are not displayed to you at all. You can optionally make the “Friends and Foes” information public… so you could see how many friends and foes an individual has.
I’d vote “no” on adding that feature. I successfully graduated from junior high school a long time ago and feel no need to return.
I’m not into foes either. I do like the karma/thumbs up/down feature though. But, then again you can PM someone too… Can you just add the feature to specific forums? or does it have to be across the board?