If all of the farmers collaborated, they could demand anything out of Rome by simply refusing to ship grain to it.

well, Rome would just reconquer them again - back than they didn't need excuses.

However today... well it actually still will be hard to do by simply doing an economic blockade - government can tax you for your land and if you don't sell your food, you can't pay those taxes.

Doesn't change that going into local politics and holding your ground is the first step. Once you have your ground, you can figure out ways to expand.