Simply uncheck SAML 2.0 under Settings > Authentication (see screenshot). IMPORTANT NOTE: Please ensure you have Username and Password (Top Line) checked before you do. This will ensure that all existing users can still log in and reset their passwords using the login page of your instance.
After that, you can ask your IT to revoke the certificate and all the SSO information in Entra assuming you don't wish to turn this back on the in the future. Or you can keep it for reference. The option will be gone from the login page and no longer show as available/possible either way.