I believe there should be no special treatment for new players. Life isn't fair, and Diplomacy is far less fair than real life. That's how the Creator intended it :-)
The only way I see to prevent experienced players running roughshod over newbies is simply to never put them in the same games. This idea works well for Chess, Go and Bridge, why should Diplomacy be different?
If new and experienced players must be mixed, the newbies require training on the rules and, importantly, the spirit of the game, before their first play. The rules of support, and to a lesser extent convoy, how to write orders correctly, and why orders cannot ever be changed once they are revealed are major sources of confusion and frustration, and they warrant a training session to ensure that at least everyone understands the game they are playing. This can take a few hours to do it right.
And even before they agree to play the game, I always give new players a standard disclaimer, that people can be very mean to each other in this game, even those who are your friends in real life, and if you're not OK with that, maybe you shouldn't play after all.
Anyone who passes both of those initial barriers to entry deserves to be treated with all the respect due to a competent player - that is, with no mercy whatsoever. Surviving and thriving in a den of lions is what the game is all about, after all.
In response to your particular concern, I'll say one thing more. For every duo of experienced players that forms a Juggernaut alliance and bulldozes the newbies, there's a "carebear" player out there somewhere who "teams up" with the newbies, and promises to help them and never ever betray them (very seductive for many newbies), and basically ends up dictating their orders to them, turning newbies into zombie players. These arrangements are utterly indistinguishable from a single player controlling two Powers.
I've played with such people. I find players who coddle (zombify) newbies are harder to defeat than experienced Juggernauts, because at least you can sell an "attack the cabal" alliance a lot easier than you can sell a "kill the newbies to drain the zombie master" alliance. Granting preferred placement to a "carebear" / zombie master player is basically my worst nightmare in Diplomacy. Please no!