Automatic re-routing is an awesome feature for most users, but it is not for everyone. A trip-planning software should allow for disabling it, if a user so chooses.
I have sent a quick e-mail to the S&T Team via its blog - if anyone reveals a way around it, it'll be them. I'd say don't keep your hopes up, but there is at least a small chance there may be some way to do it via the registry.
Failing that, the only way to avoid the automatic re-routing is to plot a route, but don’t enable driving guidance. You will not get voice prompts, which is bad, but the automatic rerouting will not occur.