The phone will automatically download the update, but not install it. To delete it, go to settings->general->storage and icloud usage->Manage Storage (there are two buttons that say this, click the top one). If the update downloaded, it should be there. Tap on it and click delete.

The other thing you can do is install a tvOS beta configuration profile. This will make your phone look for tvOS updates rather than iOS updates, which it will not install because they're not for ios. I won't post a link here, but it should be fairly straightforward to find.