How do I turn off auto-retrieve on the stock text messaging app?

I'm hoping this thread will make my question more visible. I have searched and searched and I found how to turn off auto-retrieve in the settings for the stock android text messaging app. Only one problem: That option is missing from the settings on my phone. I'm assuming this was done intentionally for AT&T so I can't use a third-party free SMS app.

I have an old plan with unlimited data that I obviously don't want to give up. The trouble is that I have my text messaging plan (200 texts) tied to it, so if I try to upgrade my text messaging I will lose my unlimited data. So I installed HandCent, but the messages are still captured by android messaging and thus will cost me once I exceed 200 texts. HandCent even has an option to make it the default messaging app, which is supposed to disable any other messaging app. But it doesn't.

Am I stuck? I realize there are other free SMS apps that give you a separate number, but I was hoping to stick with my mobile number for simplicity.