the simpliest way is just not to show rate form for the owner:
in includes/functions.php find if (check_permission("auth_vote", $image_row['cat_id'])) {
replace with: if (check_permission("auth_vote", $image_row['cat_id']) && ($user_info['user_level'] == ADMIN || $user_info['user_level'] == GUEST || ($image_row['user_id'] != GUEST && $image_row['user_id'] != $user_info['user_id']))) {