How can I stop iOS updates from downloading automatically via Wi-Fi, on my iPhone 6s with iOS 9.0.2?
Apple isn't 'forcing' downloads on you, but instead is making it convenient and easy to stay up to date on Apple software. That may sound like a spin on the point of view, but Apple doesn't want you on the latest release so they can crow to the press about update percentages.

They do this for at least two reasons: 1) it reduces their support costs, as customers migrate off older code, and 2) it provides critical security updates and bug fixes that may put your device and data at risk. 

Apple does drop support for older OS (if you have an original iPad, you know it is 'stuck' on iOS 5).

Automatic download of the updates does not mean that you are forced to take the update, it just means the update package has been downloaded to your device. Installation is still something you must approve, and kickoff. The phone will not update the OS without your input. 

You won't suffer from new features or anything in the new updates until you apply them, so other than using some storage space, there is no downside to having it downloaded in the background.

Something to consider: Apple includes many security updates within its OS updates, most of which are (sadly) not well documented in the update notice. skipping these updates may put your device, and data, at risk.