In the forum (i'm using newbb), to delete the entries go to PHPMYADMIN, choose your DB and click on the SQL tab. Type this in. Replace [THE USER ID] with the spammer's user id. Do not delete his account yet. Do it last.
Delete xoops_bb_posts_text.*, xoops_bb_posts.*
FROM xoops_bb_posts, xoops_bb_posts_text
WHERE xoops_bb_posts.post_id = xoops_bb_posts_text.post_id
AND xoops_bb_posts.uid =[THE USER ID]
Delete xoops_bb_topics . *
FROM xoops_bb_posts, xoops_bb_topics
WHERE xoops_bb_topics.topic_id = xoops_bb_posts.topic_id
AND xoops_bb_posts.uid =[THE USER ID]
To delete the comments
Delete xoops_xoopscomments.* FROM xoops_users, xoops_xoopscomments WHERE xoops_xoopscomments.com_uid = xoops_users.uid && xoops_users.uid = [THE USER ID]
Finally go and delete the user account.