Here's how you can block cellular data usage in iOS 7 on an app by app basis.
1. Go to Settings.
2. Go to Cellular.
3. Scroll down to the Use Cellular Data For section.
4. Toggle the app switches to the On or Off position.
If you have a limit on your cellular data usage, you should definitely spend a couple of minutes making sure that only the apps you want can use cellular data. Otherwise you might find that you've gone over your data limit for the month.