I doubt this is the best way of doing it, but off the top of my head I would suggest giving your
Registered Users usergroup a
Maximum Stored Messages limit of 0 (you can do this under
Usergroup Manager -> Registered Users). And leave the Moderators/Admins with the default figure.
Again, not sure if that's the best way of going about it but that's all I can come up with right now...
By the way you posted in the wrong section.