Oh my gosh, thank you! This should be pinned as a top answer. Hours of researching how to remove his with repeated pop ups and you solved it!
It is not necessary to have access to the Work or School account you want to remove. The only requirement to remove it, is to be logged on with administrative privileges on your computer.
You can logon to your computer with a local or Microsoft account that is in the administrators group, as described in the article above.