I did this in the Icehouse release of OpenStack, maybe you can map this to the Havana release:

1. log into the database (you should see > mysql in your console)

2. select the nova database:

   use nova;

3. mark the rows in table instances as deleted (that's a "soft-delete")

   update instances set deleted_at = updated_at, deleted = id, power_state = 0, vm_state = "deleted", terminated_at = updated_at, root_device_name = NULL, task_state = NULL where deleted = 0;

   <-- That 'deletes' ALL your instances! Use show columns from instances; if you want to choose another column(s) for your where-clause.

4. update the cache in table instance_info_chaches appropriately

   update instance_info_caches set deleted_at = updated_at, deleted = id where deleted = 0;

5. update the fixed_ips table:

   update fixed_ips set instance_id = NULL, allocated = 0, virtual_interface_id = NULL where deleted = 0;

Note: If the column deleted contains a value not equal to zero, then this seems to be the way to say this row is supposed to be deleted. When I delete an instance via API, OpenStack seems to choose the id as value for deleted.

Source: http://www.databaseskill.com/4605135/