Recently I faced the problem on how to remove a “onmicrosoft”-Account from my company’s windows 10 pc. It was not possible to remove it in this screen:

There was only the option to “Manage” this account. But somewhere in the depths of the internet I found the solution where to remove this account: In the same screen you find the menu “access work and school”. There you can find all accounts and you are able to delete or disconnect them. But be careful to choose the right account!