Does anyone know how to report a SteamID to Valve?
I can't find a "Report offensive user" link anywhere.
if it's cheating you need a vid
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Well..VAC take care of that, if the person is using a detected hack, they'll be banned within 2-3 weeks. If it's undetected, unfortuanately VAC do not accept videos, as people can be framed. if the server had steambans running, and you typed sb_status in console while recording, you can get them steambanned, which excludes them from several thousand servers.
I'm sure there's a way you can contact Steam and let 'em know about that player. If you don't have a video, you could just get them to survey his online activity for a while.

Dirty hackers.
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"The end is extremely fucking nigh..."
No, I believe they either need to directly detect a program running, as some less obvious hacks can sometimes look like player skill/vice versa.