I think a negative vote isn't a good concept to implement in a community because it can easily foster ill will and many tit-for-tat fights.

I'd suggest reducing the karmic value of basic 'thanks' to 1/5th of its current value, and adding a 'Useful' button\flag that has the value of current Thanks.

This way we can still pass on our 'thanks' for those friendly and entertaining posts but have the option of giving a more weighty 'Useful' post that we deem more worthy.