Regardless of what anyone says, there is ALWAYS a reason someone performs an action. "Because I feel like it," is a reason. There are psychological problems with some people, some of those people play video games, but to try to label griefers as a massive group of people with something horribly wrong with them is just plain crazy in its own right.
I'm gonna open up a can of long winded typing now, so for those who don't like words, you can just skip down below to someone else. First of all, true griefing is not cool in my opinion. If you drive your teammates off a cliff once, that's fine with me. Sometimes the randomosity is welcomed amidst the same old find kill tactics your team will be employing the whole round. However, if your purpose is to constantly kill your team, to either make sure they lose, or to make sure they don't enjoy the game themselves, that's pure griefing, and it's wrong to do, whether you believe it is or not.
That being said, a lot of people who play games put themselves into this weird state of mind where they think they MUST win, or achieve some contrived goal, and if they don't they flip out like you wouldn't believe. These people really do need to lighten up. Some games are competetive. I grant that, no problem. But when you allow the goal of winning to take over the FUN aspect of the game, you're taking it too seriously. Games are supposed to be fun, just by playing them. If you don't enjoy PLAYING a game, you're missing the point of a GAME.
There isn't a single person out there who can simply pass judgement on someone else. If you believe someone is taking the game too seriously, it isn't your right to make them stop. Just ignore them, and let them game in their own little world. If I'm griefed, I simply ignore it. Someone called me a 1000 pound biatch loser with no life today, because I said something on world chat in the MMO I'm currently playing, and they didn't like what I said. I just let it go, it's the best thing you can do.
No one should try to take your fun out of the game, but in the same sense, you are the one who lets them take it. If they are messing with you, and they don't get a reaction, they aren't going to keep doing it. They WANT you to get mad, that's their fun. If you ignore them, they'll move on to someone else. If you feel you need to take further measures, report them. Dropping to their level isn't going to help, and it's not your job to "keep the peace" either, most companies have paid representitives who are paid to pretend to do that. *cough* Blizzard *cough*
As for hacking, or things that can't be ignored that limit your ability to play, report them, and move to a different area/play a different map/the equivalent to walking away and going somewhere else. If all else fails, walk away from the game, take a deep breath, come back in ten minutes, and see if it's still happening. After all, it's just a game, and it's not worth your mental or physical health.
-Froggyman