The only way is to download the route to your phone and then stick your phone in airplane mode. It won't be able to reroute then. However, when it is rerouting it prompts you that it'll do it, you can just decline the reroute. Alternatively, you could use the app Waze and it'll give you it's route, but it'll learn from how you drive and then that becomes it's route. Also other users can add information, like hazards, speedchecks etc and it notifies you in advance of stuff that's upcoming.