You can't. People making absurdist statements like that and claiming them as "opinions" when someone rebuts them are the principle reason I stopped looking at social media sites (and I barely looked at it before then). If someone says, "The sun didn't rise today" and everyone else points out the sun in the sky, and their reply is "Well, that's, like, your opinion, man", it's safe to ignore them. Don't engage because they have tacitly admitted to being an unreasonable, delusional individual.