Command syntax looks good to me. Not sure why it fails to remove the service registrations. You can also try removing the service registration using Jxplorer(Open source LDAP browser)
Note: Take a snapshot of vCenter server before trying unregister again
1. Download and install Jxplorer on a Windows machine VMware Knowledge Base
2. Refer just step.1 and 2 to connect to vCenter
Once you have Jxplorer connected. Please expand the trees further as following:
Configuration --> Sites --> Site name/Default -site --> LookupService -->ServiceRegistrations --> Look for the service ID captured for vSphere replication and delete it