You certainly could block the IP and/or domain in your router. For instance, on my Linksys router you can block any url by editing the router's settings. However, not all routers have this functionality, for example I also have a router provided by Xfinity that doesn't provide any domain-specific filtering.
If that doesn't work, you might be out of luck. Other than disabling in app purchases, there's no easy way to get granular control over particular functions within an iOS app. You could turn the device's wifi off altogether (i.e. Airplane Mode) and lock it with parental controls, but that would turn off all web browsing, email, etc.