@controlord from the looks of it he did not hack as you cant really no clip with out sv_cheats on servers unless he owns the server the other possibility is doing the classic sit on a barrier then alt tabbed out of the game as this is not really a hack but a glitch with the game.
This is something we will most likely not ban for as he is not really hacking but just abusing a glitch, I would just report them to the server's moderators / admins