I've experienced this only once, and it was a loooong time ago. It was a girl reading a magazine at a bus stop, and it made me kinda uncomfortable so I just said "whatever" and ejected. This was during my transition into using more natural approaches as opposed to the canned stuff I'd come up with, and I hadn't learned that there's a right and a wrong way when it comes to situational openers. Yep, I actually asked what she was reading instead of using observation and wit to get her laughing. How lame.
But I digress. You're not gonna come across a girl who gives you the silent SPAM very often at all. It hasn't happened to me since then, and you're probably never gonna have it happen to you. If it does though, the key is in your attitude. If you're self-assured enough to know it's not personal and how quickly she'll stop acting like that once you show her you're not the type to walk away so easily, you'll just make light of the situation and get her laughing. Tell her if she doesn't say something within 5 seconds you're gonna go tell some random guy she really fancies him and bring him over. Stick to your word, too!