Disable inbound emails except root

We have taken the bold step of no longer providing email hosting to our customers.

I have already managed to move most people over to either Google or Microsoft however I would like to make it impossible for people to create mail accounts and continue to use emails on our server and to completely disable inbound mail. Outbound could remain, I can rate limit each account to 1 per hour which makes it fairly unusable.

I have considered turning off the exim ports in the firewall, would this be sensible? What impact would it have? I guess it could interfere with notifications for root?

Also in the package configuration I wanted to stop mailboxes from being created but unfortunately zero means unlimited. Is there any solution to this?

Thanks in advance for any help

 

Ideally I would like to disable users from creating accounts but also block the default account from being used too.