In phpmyadmin run this
PHP Code:
DELETE FROM `post` WHERE `username`="DELETED GUEST NAME HERE"
change the username and change `post` to include your databse prefix if any so if your prefix is vb_ then it would be `vb_post` if your prefix was vb then it would be `vbpost` if you have no prefix then it will be `post`
If you want to keep the posts but change the username let me know and i'll give you the query for that.