You can revoke MFA from your account yourself via your Reckon Portal account settings but it requires that you have the original app that you used to authenticate initially. Unfortunately, it sounds like this doesn't apply in your case as you've mentioned the original device is defunct.
If you don't have the original Google Authenticator used or if you need a MFA code revoked from your Reckon Portal account for any reason other than the above, there are stringent security controls around this.
You must get in touch with our Support team to initiate it as they will need to verify the account owner. If this is successful, they will organise for MFA code to be revoked. Its not instantaneous unfortunately due to the sensitive nature of information in the Single Touch Payroll process.