Whatever your beliefs, you are doing your movement a disservice when you personally attack those with differing opinions. If you believe spanking to be wrong, it does no good - in fact, it does great harm - to tell a spanker that they are evil. You are not going to sway anyone to your belief system by calling them a name, belittling them, or in any way making them feel inferior. You will drive them away. They will be lost.This has been a weakness of mine, unfortunately. But I still got time to change!
Also, keep in mind that you are not only responding to one individual. You are responding to an entire community, many of whom will judge your argument not on its merits, but on the way in which you defend it. To an innocent bystander, those slinging insults lose much credibility.
This "silent majority" should not be discounted. Most people do not comment on posts. They don't click "like" or "share" content. But thousands and thousands of them do read what you've posted. And they will either be persuaded or put-off by the conduct of the participating parties. In many cases, what you're saying isn't even important. It's how you're saying it. It's your language, your grammar, your sentence structure, your passion, and your comportment.
Be a good advocate. If you're advocating for peaceful parenting, by all means conduct yourself in a peaceful manner. If I arrived on a peaceful parenting website and the commenters were ripping each other to shreds, I'd walk away thinking those people were mad. Don't let your emotions get the best of you. Refrain from the personal attack. Be kind to one another. You can disagree without being disagreeable. You can make your points without pointing fingers.
You want a kind, peaceful world? We've got to become that ourselves. The change starts within each of us.