To delete a "ghost" instance from the Nova db:

Delete the row in “fixed_ips” table corresponding to that instance_id
Delete the row in “virtual_interfaces” table corresponding to that instance_id
Delete the row in “security_group_instance_association” table corresponding to that instance_id
Delete the row in “instance_info_caches” table corresponding to that instance_id
Finally, delete the corresponding row from the table “”instances”