Is the person a valid account that you want to keep, or one that you want to delete?
If you delete the account I think it asks what you want to do with their posts (not positive since I've not deleted any for a long time)
If it doesn't ask but you do want to remove their posts then you might have to do it with a MySQL query.
One of the others should be able to chime in here soon with perhaps a tested / known resolution for your issue.
