What can you do about brainwashed people who won't look at any reasoning?

Don't knock it until you've tried it.

They will when they are dead 

Prayer impacts any circumstances.

I was brainwashed from birth into believing a tiny American denomination was the only true Christian one in the world. But, by the grace of God, who answered a particular prayer when I was in my early 20s, God led me to Christians who encouraged me to re-think, and to re-read the Bible. Two years of intense study later, and I was able to walk away from that group, their shunning of me only confirming that they were a pseudo-Christian cult. Forty+ years later, I still thank God for His intervention, in answer to prayer. I became a Christian after I was freed from the cult.

As for the user you hint at, the same thing can be done for him as was done for me by Christians concerned at my indoctrination; prayer. Same for all the others who are clearly believing everything they read on the Internet that confirms their biases; pray for them. Miracles of grace keep happening. The spiritually dead cannot see them, of course, well, not unless God miraculously brings them to spiritual life, as He did with me.

Humanity makes progress as those types die off--one funeral at a time.