do you have phpmyadmin?

Open up the database then run the following queries
Back up your database, just incase you lose all of your attachments, or my queries have a mistake in them (they're simple, so they shouldn't

). Disclaimer: I take no responsibilty for any bad things that might happen
Code:
DELETE FROM attachments WHERE dateline < somedate
and..
Code:
UPDATE post SET attachmentid=0 WHERE dateline < somedate
if your forum is large, it might take a bit to update the posts.