How to Stop a user to send spam mail?

#1
Hi
I've been on KH for a while and happy with my VPS. Just about yesterday, I found out one of my client sending a lot of emails through SMTP using his mail account.
I tried to stop that by setting the maximum number of emails send out per domain within 1 hour is 100 (and now only 20) in WHM->Tweak Setting. But look like it did not work :-(

My question is: do you know a way (or some linkt to the tutorial) to prevent this happening in the future?
Thanks
Son Tran
 
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