Disabling Personal Messages

Is it possible to disable the PM system? I have searched but can find no way to do this. Any help would be appreciated.

You can do it with permissions.
Under Admin -> Permissions.
Modify a group
And then find " Contact members using the personal messaging system"

Thanks so much!

And then find " Contact members using the personal messaging system"

I did as you said, but cannot find the above anywhere. I have a 'guest' group and wish to disable the PM system for them. There doesn't seem to be any reference to personal messaging amongst the permissions.

Guests don't have personal messages.

Normal membergroups do.

Thank you.