@mac.com used to send spam

It's pretty unlikely that someone is actually sending mail from your account. It's very easy to forge the 'From' email addresses on emails, and spammers routinely do this so that, when they send thousands of emails to speculative addresses, they don't get the inevitable bounces from non-existent addresses - you do.

The first thing you hear about this is when you start getting bounce messages because the spam has been sent to non-existent addresses and is being bounced to you. There's no point at all in responding to it. It's infuriating but normally stops after a bit as they move on to another forged address.

There isn't really anything you can do about it: closing the account isn't really worth the hassle unless you are totally swamped, because you will have to tell everyone your new address. Apple can't really do any more than they already are about spam.

You can't actually disable the address, but if you are getting a lot of bounced spams you could set up a Rule on the iCloud website's Mail page (click the cogwheel icon at top right and choose 'Rules' ) to move all mail sent to that address to the Trash or another folder. Then you won't be bothered by it (though if it's other than the Trash you will need to manually delete it periodically).