At Admin Panel -> Settings -> Switches, the "buddy and ignore lists" switch allows your members to keep track of their friends and avoid seeing their enemies. Users can edit their buddy and ignore lists by clicking "Options" from the top menu bar, then selecting "Buddy & Ignore Lists" from the left menu. The left menu there also includes an "Open Buddy List" option which opens up a messenger with buddies displayed. Members can also choose on the "Edit Profile" page to only allow PMs or IMs from buddies, if they so desire.

Anything posted by someone on the ignore list will not be displayed. If a member can't stand another member, this is a good way to make them disappear. Normally used on discussion forums.