How to completely wipe an iphone?

This isn't super ethical, but I found an iPhone that I'm gonna keep. I turned off all location services, and didnt sign into my apple account so it cannot be traced. I reset it so there's no more numbers or pictures. However it still says its using 3G and on a plan. Is there any way to cancel the plan from the phone. Or turn the phone part of so it's more like an iPod touch? Thanks! X

You need to connect it to your computer and restore but when the thing comes up on itunes to set up from back up or as a new iphone choose new and then dont do anything. When the screen on your iphone is the setup screen. Just leave it. Hope this helped!