In Beta5...and the nightly...from menu: Tools -> Plugins -> Manage, if I click "Uninstall" on an existing plugin, the listing is immediately removed from the Manager's window, but the plugin is still active -- perhaps there should be a pop-up saying to "restart" (as there is when I install a plugin. However, if I do restart McV, the plugin is still there!
The only way I've found to remove it is to got to the .mcidasv/plugins directory and delete the plugin.