more than likely the same steps will be needed as in that link i provided.
contact one of the agents in there. they are well versed on where to go if none of the troubleshooting steps have worked.
Please review these steps and try them again, even if you've already done it:
1. Open the Apple Watch App and make sure you pick the watch you wish to cancel. There can be 2 watches with cellular active per iPhone.
2. On the main App Screen, scroll down and tap Cellular.
3. Tap Info Icon next to Sprint.
4. Tap Manage Sprint Account. (DO NOT SELECT REMOVE SPRINT PLAN)
5. Tap Cancel Subscription. The name of the selected Watch from before will show up on this screen, so you can validate it and not cancel the wrong subscription.
6. Login using Phone Number and PIN or sprint.com account.
7. Tap Submit to confirm cancellation of cellular service.
8. Tap OK after cancellation has completed
9. Now you will see Sprint - Not In Use.