This suggestion would also prevent it from happening no matter where they had the script or what site it was on. Again, few would know how, even if it's quite simple to do. However, all in all, anyone that intent on skewing the votes would surely just jump through a few thousand proxy servers and up their vote scores anyway. However, this would stop the most obvious and easy and quick methods to skew their voting scores.
Since it's so simple to do, it would be worth doing anyway. The rest of the more hardcore abusers would have to run through a proxy. So my final suggestion due to that, would be to force people to have cookies enabled and have a double-session. Different sessions that must match the same IP. This would make it so someone would either have to use a tool or specific setting to be able to run through proxies, unless they wanted to keep trying to remove cookies all the time or use a tool. That's about as good as it's going to get and it's not too bad (for the intent of the feature in question).