You cannot delete a volume unless snapshots and backups related to it are deleted/cleaned. In the code if you see this function, it is clear that it will remove snapshots first. Furthemore for deleting the volume you have to delete all the references to it. See this function in cinder.py for more understanding.

If the volume has copy on write clones Openstack will not be able to delete it. Instead it will be kept as a silent volume and will be deleted when it's snapshot and clones are deleted.

I hope this answers your question !!