emmett: this is a bad idea for 2 basic reason,there are others:

1-many users use public library,university or community center computers,thus many on one IP address.this would unfaily block many users.
it is surprising how many there are.take a poll fencer just for curiousity.

2-this unfortunely will NOT stop advanced cheating methods..there are many,I reframe from doing so.
but many dont care that cheating is a hollow victory.
to make yourself feel like you have acheived something by cheating,is the calling card of an insecure,dysunctional personality.