I saw a post on the VMTN forums the other day about unregistering a vCenter plugin. The user had a bad installation of an early preview of NetApp's VSC utility. After uninstalling the plugin, the user was still unable to unlink the plugin from vCenter. There is actually a pretty simple solution to this problem which can be accomplished by using the vSphere MOB.
Here are the instructions to remove a specific plugin/extension from your vCenter server:
1. Point your web browser to your vCenter server: https://your_vc_server/mob and login.
2. Click on content:
The following operation can also be performed using a script, here is a vSphere SDK for Perl script that does just that: pluginExtensionManager.pl