Pushed it hard yesterday during a spirited 2 hr 80 mile drive across the Texas Hill Country, reaching 100mph at times, 90% in Sport or Sport Plus. Dropped the range some but not a lot.
You should be able to just cancel it. Have you read the contract? When we got our Defender last year I got a discount on the price of the car if I bought a protection package like this one, plus a lower finance rate, so I did, and then a couple of months later I called to cancel it.
When the message pops up and gets grayed out, go to the AC menu (not the shortcut in the trackpad) and once there turn it off, and try again. This worked for me.