Why not challenge the person by contributing a source which gives an opposing opinion, explain what that opinion is in simple words, then say something like "I feel this and this better reflects the reality of immigration". A reply like this is far more constructive and allows the person to re-evaluate their position,
So I should have written something like "your claim is entirely false, anyone who wanted to make such a ridiculous claim should have had the intelligence to look at a neo nazi rally before they made such a silly statement as it is clearly contrary to reality and entirely unsupportable by fact"