For example, a person can say something like "taxes are too expensive" and another user might just dive in and disagree, failing to realise that we come from different parts of the world and how things are in one country or city may be different from the other. Therefore, our views will be different, and that is fine because that is how we can grow or get better.
Then prove him wrong by show the tax bracket in your country are high, why make it complicated? 
If they can't accept your proof and being jerk just in order to have argument against you, you have an option to reply his message or not. What I know people who already mature can make decision and focus to spend their energy on important thing, not just because seeking for validation.
Disagreement is normal, it's not even rude at all if other people's opinion are correct.