DT is right, it's quite hard to know if someone is really creating multiple accounts to upvote things, because some people use the same computers. Usually you can't tell the difference between different computers on the same network, e.g. at a school or college computer room.
But gaming the system is pretty hard. There are limits on number of actions per IP address anyway. You'd have to keep logging in to different accounts and finding posts to vote etc etc, and you won't have anything to show for it besides a few more points.
If anyone wants to waste their time messing around like that then whatever. It's actually MUCH EASIER to simply post good answers and ask interesting questions than try and cheat.