Assuming you mean a rating given by selected members and not received by: (if I'm wrong just replace user_id with rated_user_id)
Code:
delete from dark_postrating where user_id in (1, 2, 3) and rating = x
Where x should be replaced with the rating id and '1, 2, 3' with a list of user ids.
You'll need to run Recount Ratings after, and as always be sure to take a backup