If the system is not serving data to any clients, you may simply run "halt -f" on both nodes if the system is HA.
My suggestion would be to leave it turned off for a month to ensure no problems are caused, and then power it on again without network connections, and run disk sanitization per the instructions at https://kb.netapp.com/app/answers/answer_view/a_id/1034477/loc/en_US#__highlight
To enable disk sanitization on a FAS2020, you will need to contact your NetApp partner and request a license code based on your system's serial number. There is no charge for this code.