I created a trigger, and that seemed to work, here it is below:

CREATE TRIGGER t_UpdateMemberStats BEFORE UPDATE ON member_stats
FOR EACH ROW BEGIN
IF NEW.news_comments < 0 THEN SET NEW.news_comments = 0; END IF;
IF NEW.forum_comments < 0 THEN SET NEW.forum_comments = 0; END IF;
END;

This seem to prevent negative values

-Paul