Okay, just to clarify again, even though it's been said:You cannot set up a blacklist / whitelist for Airport networks.
That being said, I gave the exact way to prevent the wife from 'accidentally' connecting to other networks in my post.
Scott, you're right, it does sound like you're on a PC help line. I used to work tech support for Dell, and you sound like some of the customer's I had.
Cust: "I want to do X, because Y is causing problems."
Me: "Hm. Well, X isn't possible, but here's how we can
Cust: "No! You aren't listening, I want to do X!"
Me: "Sir, the problem is Y, though, right?"
Cust: "Yes, and I want to fix it by doing X."
Me: "As I said sir, X is not possible. However, we can fix Y by..."
Cust: "You're not listening to me!!! I want to talk to your supervisor!"