How can I stop an incoming text message while I'm on the phone?

I don't think you can. But (apart from hearing a beep in your ear), why would you want to particularly?

You can use Do Not Disturb while on phone calls to silence the incoming alert. However, this will silence all alerts until you turn the feature off. Here is a article on the feature if you have any questions, http://support.apple.com/kb/HT5463