Accidentally deleted a spammer before deleting his posts, no it doesn't seem I have an easy way of getting rid of his posts (over a thousand) since he doesn't show up in the admin CP and when I try to do searches for his username in the regular search bar to do inline moderation nothing shows up either.
Do you allow guest posting?
Do you have any other deleted user who's posts you may want to keep?
If you answered No to both the above questions then you can run this query in phpmyadmin to remove the posts:
DELETE FROM `mybb_posts` WHERE `uid` = '0';
You might need to run recount and rebuild in admincp >> Tools & Maintenance after you run the query.