I tend to think that dogmatic opinions are not changeable via online discourse. You cannot rationalize someone out of these positions—that kind of change has to take place in person with some semblance of real human care and connection involved. So, I try not to post with the intent of changing the mind of someone already committed to a dogmatic opinion. That said, I think it can still be fruitful to reply to these people occasionally in situations where I think other people reading the thread might benefit from the information. My dad has one particular troll on his Facebook friends list that he occasionally replies to just to give the troll’s wider network an alternative view to consider if they feel like it.