Here's how you can block iTunes Radio from using your cellular data to stream music. This will force it to use Wi-Fi only to play music on your iPad, iPod, iPhone or iPad mini.
1. Go to Settings.
2. Go to iTunes & App Store.
3. Toggle the Use Cellular Data to the off position.
4. This will prevent all Automatic Downloads, iTunes Match and iTunes Radio from burning through your cellular data.
iTunes Radio is a great service, but you should be careful not to let it burn through vast amounts of cellular data bandwidth.