At one point I had a bunch of notebooks on my OneNote Online Screen, with seemingly no way to remove all these old, non-existent notebooks from this view. Many of them no longer existed and were left over from various tests or MySites that have since been deleted.
The fix for this is very easy, but since it eluded me for so long, I thought I’d write it up. Hover your mouse over the notebook you want to remove from your view, and wait until the X shows up in the right-hand corner. Then click that to remove.
This does not delete a notebook – it merely removes it from your online view.
Note, this feature did not work in Firefox on the Mac, which was one reason this simple fix eluded me so long.