Instances can be ended with the euca-terminate-instances. It accepts one or more instance ids as arguments.
$ euca-describe-instances RESERVATION r-9puzwes7 book default INSTANCE i-00000004 ami-6683ba18 10.0.0.2 10.0.0.2 running ken (book, nova-controller) 0 m1.tiny 2011-07-11T00:10:42Z nova # euca-terminate-instances i-00000004 # euca-describe-instances
Simply shutting down or powering off an instance does not terminate it in Nova. You need to actually use the euca-terminate-instances command to release their resources. This is different behavior than you may be used to from Amazon EC2.