Online, there is no way to handle it. People are going to impose their emotional responses on your text no matter what you say. Unfortunately, that’s the nature of anonymous, text-based interactions; there’s no way for you to win. The phrase “calm down” is being used to invalidate whatever you say. The whole point of using it is that there is no way for you to win other than to not play the game at all. So I’d just completely ignore the statement.
In-person, they’re attempting to do the same thing (invalidate you) and derail the conversation. Instead of talking about whatever it was you were talking about, now the conversation is about you and your supposed emotional reaction. So you’d kind of do the same thing: ignore the statement and redirect the conversation back to its original topic.