It's just so hard to come up with a foolproof method. I mean, if I game is popular, it's going to attract a lot of people, and out of those people there's going to be a few idiots that just want to screw it up. It's something that you've got to deal with, something that you've got to live with. It's difficult though, trying to find a fix for it. It varies with the cheat.
Some cheats are really obvious to spot.
But some of them cause a lot of paranoia. It really ruins the whole atmosphere and play of the game. If you see that kind of thing, you can try to vote them off. But I don't know, other than that, it's really kind of hard to tell. If you find a cheater, I think Half-Life has a way of banning people.