Used to allow only those in the room lists to join the room. Everyone else will be kick/banned as they join.
Example:
/msg chanserv set #support restrict on
A good command to keep a room completely private and control who enters. The downside is that you must add an entry for everyone whom you want to allow to join. You cannot just invite someone in when you want to, you must first at least give them UOP access. Which also means that everyone allowed into the room can look at lists and see whom else is allowed in.