I have a few (former) Operations Manager installations in the list. Anybody has an idea how can I remove them from UM?
I'm not sure this will remove OpsMgr from the Usage meter, but the most probable reason it's in the list is because there's still a reference to in your VC(s).
So first step would be to remove these using the MOB: https://kb.vmware.com/kb/2036389
Delete the VC associated with OPs manager in Usage Meter, By doing so, automatically Ops manager is removed from UsageMeter.