There seems to be no way to remove a plugin that was previously installed.
How might I do this?
Currently we don't have an option in the UI for this, but you can remove the plugin from the file system. Make sure you stop Greenshot before you do, otherwise the file is in use and can't be removed,
This is how you do it:
Look in the installation directory and find the plugins directory, in there all plugins are "installed". You can delete the directory for the plugin you no longer want so Greenshot no longer finds and loads it.
As there are no references in the registry there is no "housekeeping".
There is also a way to exclude plugins from loading by placing them in the ExcludePlugins property of the Greenshot.ini, but you'll need to know the name of the plugin you want to disable.
Method 1 worked, thanks!