Despite many complaints and requests, there is still no clean, easy way to uninstall plugins. The best way I know is using the PlugInManager command to find directory/folder where the plugin was installed. Next, I quit Rhino and delete the folder. Finally, I launch Rhino and verify that no trace of the plugin is left.
Sometimes you need to edit the registry and delete the entry for the plug-in as well…
Some plug-ins can be and should be uninstalled from the Windows OS Control Panel.