Some back ground about the issue :
I removed a plugin from Add/remove Programs of vCenter server but still it was showing up in the vSphere client. I was able to enable/disable it but could not remove it from the list of available plug-ins in Plug-in Manager. This becomes an issue specially when you don’t want people ( vmware admins and non-admins) to see any such plugin when they connect to vCenter server from the vSphere client.
To resolve the issue I followed this VMware KB article, which worked like a charm:
Please also refer to these links for more info on How to Remove Plug-ins from vCenter :