Confluence doesn't allow the removal of user who has created any content. The standard method for removal is described in Removing a User. If you want to delete the content as well, the SQL is described below. Locate the content created by the user using the queries:
SELECT * FROM CONTENT WHERE contenttype = 'COMMENT' and creator = '<Username that you wish to remove>'
and
SELECT * FROM CONTENT WHERE contentid IN ( SELECT DISTINCT pageid FROM CONTENT WHERE contenttype = 'COMMENT' and creator = '<Username that you wish to remove>')
Run the same DELETE commands after verifying the content is indeed content you wish to delete.