Maybe the block with the "for this block" folder is in the thrash? Have you tried emptied it?

Also, if you can access it, you should see the ID in the url. A dirty fix would be to call Delete on that ID directly:

var contentRepository = ServiceLocator.Current.GetInstance<IContentRepository>();
            contentRepository.Delete(new ContentReference(123),true);

Thanks of the reply.

The containing block has already been removed from the thrash. I thought that any blocks in "for this block" folder would be removed as well but I guess that is not the case.

Yes I can get the block ID since I can access it in edit mode via admin mode remove block type. I suppose that might be ok this time but it would be a really good to be able to remove a block in some other way.

There is a scehduled job "Remove UNrelated Content Assets" that will remove all unused content asset folders ("for this page/block") and everything ther in.

I was not aware of that. You learn something new every day! :-) My dirty fix should be ignored then

That worked like a charm. Thanks Johan.

Just letting you guys know that "Remove Unrelated Content Assets" did not work for me. I had a few blocks that I removed from code, and because I had refactored content types that used to contain content areas which accepted the removed type, some of the blocks are stuck in limbo. It is unacceptable to be required to remove content by their ID's. What really needs to exist, is a 'remove this block' when you're in block edit view.