So an obvious hacker comes in. Do not yell, "OMFG HACKER!" It's like yelling OMFG Deer! when you're hunting.

Instead, record a demo.

Recording a demo:
Go to the console (if you don't know how to this this...)

Type: record demoname
Type: status

Go spectate (if there is no spectate function kill yourself by typing: kill [in console])

Spectate the suspected person.

Do this for awhile. More evidence=better.
When enough evidence is gathered...

Type: Stop

Upload the demo on yousendit.com or something.

Tell the server admin or try some higher things like steambans.com or vac...(VAC probably doesn't take demos because, you know, it's working so well).