How do we get these empty collection libraries to go away? If there is nothing in it then it’s pointless to have.
You can delete the collection like you delete any other thing in that list (e.g. movie. show). It’s kept around on purpose so that if you’ve configured it with custom poster, etc. and are just moving things in and out, it doesn’t disappear out from under you 
How? I’ve looked and not see the option.
Due to a bug in the web app, you need to have ‘Allow deletion of media’ activated.
Otherwise the ‘Delete’ menu item doesn’t show up.
(this bug will get squashed in future a web app update)
https://support.plex.tv/hc/en-us/articles/201273953-Collections
this got me curious, and i too am not able or see a way to delete a collection. I’ve attempted to delete the title and sort title, but that just moved an empty place holder to the top of my movies.
