I don't know if this has been addressed in recent versions, but I can recall that if you only have one or two users that need to be silenced (!) you can set their status as 'child' or 'under 13' etc.
As per
this thread.
Hopefully someone will correct me, but at least you get a short term solution
Of course if you
want them to get PMs, they can still switch this option off in their profile.
