Internet Argument Techniques
From the boys at Cracked. I'm putting it here because, you never know. Thankfully I can say we don't do most of these.
How to Combat These Techniques
The important thing to remember is that you are dealing with a person who has essentially placed a twig on their head and expects you to believe they are a tree. Armored in that level of "confidence," almost anything you say will be construed by them as evidence that their tree disguise is working, so you will have to be very specific and to the point in addressing exactly what they're doing and why it's not fooling anyone.
Subtlety and sarcasm are great tools but there is no way in hell they will get through to this poor sap. Unless you're just toying with them for your amusement, the best way to put an end to it is just to be direct. Point them to this guide if necessary, explain that you know what they are doing, and that it is so obvious it can be seen from space.
Chances are they will just end up placing more twigs on their head, praying fervently that this will succeed, but at least you gave it your best shot.