Denying PM ability of usergroups

[Sykoi]

Lately I've had a lot of spammers on my forums, PMing every user with spam... I have set "Maximum Recipients to Send PMs at a time:" to 0 for the groups these users are in, but yet they are STILL able to PM.
Why does this not work? How do I make sure they are unable to PM?

[Wayne Luke]

Turn off PMs for those groups. Set "Maximum Stored Messages" to 0.

[Sykoi]

This means the user won't be able to send out PMs?

[Wayne Luke]

Yes. The description for the option says "Maximum Stored Messages:If you set this to 0 users from this usergroup will not be able to use private messaging."