And the most frustrating part is it's a no-win situation: either you spend time refuting only for the story to change slightly (or more arguments back), or the poster assumes silence means agreement, enforcing their opinion and sharing similar later.
Direct refutation of what they have said suggests that you are open to discussion in light of further evidence, and invites people to find new arguments; whereas, saying that you fundamentally disagree makes your position much clearer.
Providing the reason might, of course, provoke other fights, or be embarrassing.
I believe it is actually unethical to post things you haven’t verified. By doing it you are actually making the world demonstrably worse.
Block, unfollow, defriend etc ... I can be friends with someone in real life and nit have their every nonsense thought gumming up my Facebook feed.