Hmmm... well.. write and wall generally do not operate across a network. Those programs send messages to tty's or pseudo tty's.
Maybe it's a different utility that people are using?
In general a user can't use wall because they don't have rights on all the tty's by default (unless rights have been opened with mesg...). With regards to restricting tty access for this kind of messaging do a man on mesg.
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