For the repeated prompts on your PC, you may be able to remove the old account from Windows without deleting the Microsoft account itself. Please check:
Settings > Accounts > Email & accounts
and also:
Settings > Accounts > Other users
If the compromised account is listed and it is not the Windows account you are currently using, select it and remove it from the device. Removing an account from Windows removes its sign-in information from that PC; it does not delete the Microsoft account itself.
https://support.microsoft.com/windows/security/identity-signin/manage-user-accounts-in-windows
If Microsoft 365 applications such as Word or Excel are still asking for the old account, open one of them and go to:
File > Account > Sign out
Then close all Office applications and sign back in using your new Microsoft account. If the old address continues to appear afterward, also check Control Panel > Credential Manager > Windows Credentials for credentials associated with the old Microsoft account and remove only those entries related to that account.
https://support.microsoft.com/windows/security/credential-manager-in-windows