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 didn't sign into my apple account so it can't 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?

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 don't do anything. When the screen on your iphone is the setup screen. Just leave it. Hope this helped!