In the Forum_Forums table there is a column named PostsToModerate I believe, this is where it gets those figures from. When you delete a post, that count should be decreased by one but it seems there is a bug somewhere else around this. I know this is an issue and I have to look into it further (I have seen it here) but I have never seen it actually show the negative numbers.

You could do that, but it would be best if you created a query to do this that did something like:

"DECLARE @@PostsToModerate int
DELCARE @@ForumID int
SET @@ForumID = X" <- X = the forumid you want to deal with first, you will need to cycle
"SET @@PostsToModerate = (SELECT COUNT(POSTID) FROM Forum_Posts P
INNER JOIN Forum_Threads T ON P.ThreadID = T.ThreadID
INNER JOIN Forum_Forums F ON T.ForumID = F.ForumID
WHERE IsApproved = 0 AND F.IsActive = 1 AND F.ForumID = @@ForumID)
UPDATE Forum_Forums
SET PostsToModerate = @@PostsToModerate WHERE ForumID = @@ForumID"

I just did that statement from memory, but it would be very similar to that. You could also write a cursor statement that would loop through each ForumID.