 To delete a source from your main library, right-click on it and select Move Item to Trash. Then you'll be prompted to confirm the deletion. Note that there's a separate trash can for each of your libraries if you happen to have more than one. And if you change your mind, you can go to the trash, right-click on that item, and choose Restore to Library. Then if that item was also put into any folders, go Return there too. If you only want to remove an item from a folder and not delete it or send it to the trash, right-click on it and choose Remove Item from Collection instead of Move Item to Trash. This way, it'll still be in the main library and any other folders you happen to place it in.