You can use the nova reset-state command to manually reset the state of an instance to an error state. You can then delete the instance. For example:
$ nova reset-state c6bbbf26-b40a-47e7-8d5c-eb17bf65c485
$ openstack server delete c6bbbf26-b40a-47e7-8d5c-eb17bf65c485
You can also use the --active parameter to force the instance back to an active state instead of an error state. For example:
$ nova reset-state --active c6bbbf26-b40a-47e7-8d5c-eb17bf65c485
Hope this will solve the problem.