You should be able to manually delete the user row.
Note that in Administration > Site Settings there is a checkbox for "Really Delete Users?"
If not checked then users are flagged as IsDeleted rather than actually deleted. If unchecked then deleting a user will really delete the row. However if a user row is really deleted then any forum posts or webstore orders or other data previously attached to the user will be orphaned but not deleted.
Hope that helps,
Joe