Hello Giles Ison,
You should have a global administrator or an admin with the necessary privileges, immediately remove the user’s access to your organization’s resources or re-register MFA for the account.
- Sign in to the Azure portal.
- On the left, select Entra ID > Users > All users.
- Choose your account->select Authentication methods and click "Require re-registration for MFA".
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Regards,
Abiola