It should be possible to un-rate a post completely after picking a rating. Currently you can change your rating to something else (unless you rated Coin or Diamond), but not remove the rating entirely. Kind of annoying that it's a one-way street.
Wasn't it like that on oldpunch?
Asshole.
Edit:
I take that back
Edit:
Suggestion: If you click on the rating icon you rated, it should withdraw.
That doesn't mean it should stay.
It probably will. I remember it being discussed on the other forums and from what I recall it was decided that it wasn't nearly a big enough issue to bother with.
Admittedly it's one step forward from oldpunch, in that you can't go to click reply then accidentally click a rating instead, since ratings now require you to open them first.
Alternatively, give us informative so we have a suitably neutral rating to change to.