From Elendor
Blocking +mail messages from arriving in your mailbox is accomplished by
setting a Maillock (@lock/mail) on yourself. See HELP @lock and subsequent
texts for the syntax of the @lock command and the various types of locks
you can set.
Example 1> @lock/mail me==#0
Stops you from receiving +mail from anyone. (Literally, the lock says you may
only receive +mail from #0. Since #0 is never a player, this means you
cannot receive any +mail at all.)
Example 2> @lock/mail me=!=*playername
Stops you from receiving +mail from the named player.
Additionally, you may also set a MAILFAIL attribute on yourself. The evaluated
text of this attribute, if any, will be sent to the person trying to +mail
you.
Example> &mailfail me=Sorry. I'm on vacation until September.