You're supposed to use the key ID which should have no spaces or ampersands. If you're trying to use the name it won't work to begin with.
I was just following the steps in the KB - VMware Knowledge Base
Step 4 - Select and copy the name of the plug-in you want to remove from the list of values under Properties
I see William Lam suggests clicking on the item, and copying the value from the key field, which appears to have the same value as the name from the previous screen https://www.virtuallyghetto.com/2010/07/how-to-unregister-vcenter.html however, if I click on mine, I get an error
I'm trying to find the file through a recursive search of the VCSA via WinSCP, since browsing through the plug-in directories I saw referenced in various KBs didn't turn anything up. I already have an SR open about rapid memory leakage in VCSA, and I thought maybe this bad plug-in was the cause. I'll just ask my support engineer about it during the call.
Did you get this resolved? I have come across exactly the same issue on a vCenter I am cleaning up before upgrading.