So, what do you do in a debate, when you figure out, that this is not a debate, this will be you explaining the basics to a person who thinks they know everything?

Decide if you have the time. Decide if you want to put fort the effort even if you may get no return on investment for that time.

Case by case basis. Unfortunately some of those people you can't help too easily. You can do what I do sometimes rather than debating them...

If you're going to have to talk about basic things and foundational concepts that seems better as a post than a reply. Why? At least then other people might learn something from it, or someone that is not them that can poke a hole in it and knows what they are talking about might respond. :)