I usually try to explain again/better what's going on in case there's some kind of misunderstanding. After that, if the person still doesn't believe me, I tell them that's their problem and I don't feel the need to prove anything to them. If they insist and they're being less than respectful like your friend sounds in your post, then I have only one thing left to say:
"kthxbai"

...and *freak* goes offline.