If Starter Content is added to a newly created project and then the Starter Content is deleted, upon re-opening the editor the Starter Content files will return to the Content Browser.
Workaround: Go into windows explorer and delete the Starter Content Folder
1. Open Editor (new project with starter content)
2. Open a new empty level
3. Delete starter content folder from content browser
4. Save empty level
5. Close editor
6. Re-open project
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-15929 in the post.