Can I hide user accounts from contact lists?

I'm dealing with a Tenant where for unknown historical reasons users have been given multiple accounts under the same name, but with different mailboxes and different email addresses (same name, different domain).
I'm merging the mailboxes together, and aliasing the various email addresses onto the one account, so they only have to deal with one account going forwards.
Until the users (and myself) are confident that everything that needs to be transferred from the now defunct mailboxes has been copied correctly I need to keep them there, but effectively inactive, so I gave them another email address which has never been publicised (actually using a spare domain already allocated to the tenant) thinking that it would be safe as noone would be sending emails to an address that never existed before.
This all seemed to go swimmingly, until people said they had not received email and I saw they had been sent to the "secret" addresses. What I didn't count on was Microsoft 365 "helpfully" adding these addresses to people's contacts (I'm guessing through the global address book?), so when people (at least people within the tenant) send a new email and look up the details of that person they may get the entry which has the email address I don't want any new emails going to.
I've set up forwarding to make sure people receive all their emails in their current mailbox, but I'd really rather they weren't ever sent to the "secret" mailbox in the first place.
So is there a way to hide these defunct mailboxes? They are not supposed to be used to send or receive emails at this point, they're just being kept as a backup until we can verify the contents have been merged successfully into the current mailboxes.