How to disable a user account that is spamming big time

[Tzvook]

Hello All
One account on my server is spamming (right as we speak) ... it may be an issue with his website, though I seem to got over that, spamming continues (10,000 mails @ 2 hours or so).
it is being sent as a user mail something like vu2038@my-main-domain.com (not the username@his-domain.com ) ?
I know there is a command-line for disabling this account from sending mail, but don't know ecsactly how to ...
Help is badly needed here ...