When you are done with an instance, you can tear it down using the nova delete command, passing either the instance name or instance ID as the argument. You can get a listing of the names and IDs of all running instances using the nova list. For example:
$ nova list
+--------------------------------------+-----------------+--------+----------+
| ID | Name | Status | Networks |
+--------------------------------------+-----------------+--------+----------+
| 8a5d719a-b293-4a5e-8709-a89b6ac9cee2 | my-first-server | ACTIVE | |
+--------------------------------------+-----------------+--------+----------+
$ nova delete my-first-server
