How do I keep my spouse from the locked mailbox?

Excellent answers! Have a P.O. Box for a few years now. Nothing of importance comes here, but we also have a locked box.
Sometimes I mail a card to my hubs here, just for fun.  He never opens it!

Set up a mailbox just outside the door, convince him that tis is where mail is being delivered now and stuff it with junk mail and fake mail daily.

Excellent answer FF. Since you can't remove the mailbox. That should work. You can also ask about hardship mail delivery where the mailman brings the mail inside.

That's a tough request, as some people feel their life revolves around the mailbox. My sig other does that, like the mail is going to disappear if he doesn't run out there immediately :P

Only thing I can think of, rent a mailbox at the post office or UPS office for important mail. That way, your hubby can gather the mail from the townhome mailbox cluster, and the mail would be mainly junk mail and catalogs.