some user always send too many spam emails

how to disable them?
I have setted "Daily E-Mail limit per DirectAdmin User" to 50,but I still got messages which said that the user had sended too many emails beyond the limit number.

It could be a couple of reasons. If the user is a spammer, why not suspend the account?
If you don't want to do that, you should investigate why he is sending out so much mail, is there a trojan or a spambot or is his account and external party's are abusing his mail account?