How to delete workflow process?

We have thousands of open workflow processes, years old, from early days of implementation. Even with admin rights, we can't seem to purge these things. Attempting to delete an instance gives us an error;: Workflow Process cannot be deleted. It is used in Worfklow related_id 8EE630D7C2D543..... This reference is pretty useless, at best inconvenient. Is there a way to purge and to hell with any relationships? The org has stopped using workflows completely just because there is so much junk in the system that they can't get rid of.