If you don't know how to record a demo, take at look at this thread:

http://war-lords.net/hacker-abuse/how-to-record-a-demo/

Then you can use this thread as guide:

http://war-lords.net/hacker-abuse/reporting-hackers/

Or if the player left, you can try this:

http://war-lords.net/hacker-abuse/how-t ... -promptly/

If you cant find the demo, at least give head admins the time the player was on and what was the issue.