Be careful.
For some reason my phone decided to update 22 Apps while of WiFi, managed to use 1GB+ of data in a day.
You can turn off cellular for the App Store in Settings > Cellular and toggle off App Store. But that kills the use of looking for apps and such while on cellular.
The other thing you can do is go into Settings > iTunes & App Stores and toggle off Updates. This still stop the App Store from auto updating apps in the background or when launching the app.