Unlink Hotmail and O365 Accounts

Hi

We have a user that has inadvertently linked their personal hotmail account to our work O365 account. The result is that the default sending address for any emails is the hotmail rather than the work O365 account. I have deleted any references to the account from profiles but it still keeps appearing. I have also looked on our O365 tenant and can see evidence of the personal hotmail account in any of the settings.

Could someone please let me know how we can unlink these accounts.

Thanks

Alistair