Apples new feature Continuity works by linking devices that you are logged in using the same Apple ID,

That means that any of your iOS devices or Mac, that you have set up with the same Apple ID will be able to pick up calls from your iPhone. 
To prevent incoming calls from forwarding to any of your other Apple devices like your Mac or iPad you can disable this features on your iPhone.

Here is how you do it:

On your iPhone go to Settings > FaceTime, and then toggle iPhone Cellular Calls to off.

Info:

Continuity works by linking devices that you are logged in to using the same Apple ID. This is the login that you use on your App Store accounts or to verify your iCloud.
That means that any Apple device or Mac that you have your Apple ID set up with, will be able to pick up calls from your iPhone.
Switching this setting in FaceTime to “off” will break this connection for all phone calls.