How to find/stop a spammer?

I have been informed by my dedicated provider that someone is using my server ip for spamming purposes.

This is my first server so i am still new to it all. How do i go about stopping this activity? Can i find out who it is? Is it likely to be a customer of mine?

In Plesk (uses qmail) i have the mail settings set for pop and smtp authorization. I have just noticed that my /var directory is at 100% (about 5Gb), this was just in the single figures before!

Please help!

 

 

 

 

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