- Follow steps 1-4 of @JonRHerring's instructions above. At this point, your OneDrive and SharePoint processes should be stopped.
- Go into C:\Users\PROFILENAME\AppData\Local\Microsoft\OneAuth\accounts
- Find the account ID for the business account that keeps syncing. For me, there were two files in there. One was for my personal account and one had information for my long-deleted (over 1 year) business account. They are JSON files, so you can open and read them. Save this ID and the login of the account found within the JSON file.
- <etc>
Excellent find! This OneAuth folder must be where Office keeps a reference to all Microsoft accounts that have ever been signed in, and/or this folder consistently gets forgotten by the cleanup procedure when you remove an account.
After following your steps, I did find two more references to the ID of my old account left in the Registry:
HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\CDP\ - LastKnownOSBuild<ID>
HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\SettingSync<ID>\ - no values
Btw, to find file system references to your account ID, I recommend Everything Search. It instantly found all files and folders where the name contained my ID, even hidden.