It’s quite simple, the only thing you have to do is access the classic portal, https://manage.windowsazure.com > Settings > Administrators, select your account in the list of administrators and click on the Remove action.
This action warns you that you‘re going to be removed as co-administrator of the account.
Once you confirm the action, the portal will need to refresh the page to complete the changes.
If it was the only subscription that you managed, the portal alerts you that you don’t have an account to manage and the session will be closed.
It is true that those subscriptions remain on the new portal, but they are no longer operational for your user and the product team is working to remove this information.