This is untested, but the following query should delete all the pointers that are at least seven days old:
DELETE FROM w3t_Posts
WHERE B_Status='M'
AND UNIX_TIMESTAMP() - B_Posted > 7 * 24 * 3600
P.S. I'm not sure if any other tables need to be updated at the same time, though. The board post counts, at least, would need to be adjusted.
