If they're going to keep it, then I would suggest going to something like Amazon. Rate 1 to 5, and present it as an average, with the number of ratings, something like:

Score: 4.7 (344 ratings)

Currently, there's no real way of knowing how many ratings there were, which makes it a little hard to know what it means other than more is more, or less is less. Knowing you couldn't make huge swings in the average might cut down on the silliness of someone rating a person's every post just to see the number move.

I kind of agree with Taildrager that the negative ratings don't serve much use, and invite childishness. Also, why would anybody ever give something a zero rating, since it does nothing? (As opposed to giving a 3 on a scale of 1 to 5, which does have an effect.)