[Andreas]

lock tables post, thread, forum write;
delete from post, thread where post.threadid=thread.threadid and thread.forumid=1234 and dateline < unix_timestamp(date_add(now(), interval -1 month));
delete from thread,forumid where thread.forumid=1234 and dateline < unix_timestamp(date_add(now(), interval -1 month));
unlock tables;

This should prune all posts & threads from forum 1234 that are older then 1 month.