@TimoSjösten
Click on the item details for the feature service that refuses to delete,
Scroll down and click View above the Service URL box:
Confirm the “Has Views” property is true:
Click the Views link, just below Has View: true
The views page will show the Views that the system believes exist:
Click the link for one of the views and you should get an invalid URL
Back on the views page, copy the full name of the view: for my example, the top view full name is “WaterMapNotes_editing_490a541df35a4ca7a2e02e3d2ce6795d”
Back on the Service Item details page in ArcGIS Online for the Feature Service that won’t delete, scroll up and click Create View Layer:
Paste the copied name into the Title box:
Click OK to create the view accepting all other defaults.
After the view is created, delete the view.
Repeat the process of creating a new view with the old name and deleting for each view that shows in the views page:
Now you can delete the feature service.
Todd