[@_ck_ Participant 16 years, 2 months ago]

This will show in you in PHPMYADMIN all users older than at least a month who have never posted:

SELECT ID,user_login,user_registered FROM wp_users

LEFT JOIN bb_posts ON ID=poster_id

WHERE user_registered<DATE_SUB(CURDATE(),INTERVAL 30 DAY)

AND poster_id is NULL