Hi Mr. P. I'm Greg, 10 years awarded Windows MVP, here to help you.
The place to start for this is Settings > Accounts > Access Work & School. Find and highlight the account to Disconnect. Now you should be able to remove it. Make sure it's removed there, in Settings > Accounts > Family & Other Users, in Settings > Accounts > Email and other Users. Restart PC in between each operation.
To turn off Shared Experience nag messages:
https://www.brucebnews.com/2019/09/how-to-get-r...
https://www.tenforums.com/tutorials/109989-enab...
Feel free to ask back any questions. Based on detailed results you post back I will have other suggestions if necessary.
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