In-app purchase popups

I have a iPhone 3Gs, iOS 5.1

I recently downloaded an app called Small Street and was checking out what they had for in-app purchases. I had to press buttons to check for a price on their items and now, a confirmation popup keeps popping up for me to "confirm my purchase". Unfortunately, it doesn't stay on the screen long enough for me to cancel it. It pops up for about half a second and disappears. It does this almost every five minutes when I play the game and I can't cancel it because it only stays up for less than a second before it's gone again. I'd like to know how do I stop this confirmation popup. I even tried to switch the settings to disallow in-app purchases but since it was already active, that didn't work.

How do I stop this popup? It is very annoying.