Emailing or private messaging you? If he is actually emailing you then block that user in your email. If you mean private messaging then leave the next conversation and then you will get the option to ignore any future conversations from that user.
For some reason I don't understand, when looking at a member's permissions, EVERYTHING is on NO. Yet he can still use the forum freely.
I'm trying to DENY this user from using the conversations feature. Is my only option to then choose NEVER? And is it irreversible? Can I later reinstate the permission?