By Ken Burkhalter updated on 11/16/2009
There has been much disappointment with AT&T for not enabling tethering the iPhone to your laptop for Internet connections.
To remove the Tethering capability, go to Settings>General and scroll all the way down to the bottom of the screen.
There you will see a line that says Profile and it should contain "US AT&T" in it on the right side.
Tap on the Profile entry. It will open up a window with a description of the Profile and a Remove button (image to left).
Tap on the Remove button and when it finishes you should be back to your original (factory) default profile. To be sure, I would also suggest doing a Restart (on/off sequence) on your iPhone too.