I'd say your original version is slightly more correct, because the emphasis is on
are not as opposed to
you are (although I'm sure that either is appropriate).
Alternatively, you could have stated simply
you are not. Or, perhaps,
you're'nt?
In any case, I would have worded the message in a polite manner, rather than going the obnoxious route. I believe this would have been more effective.
Remember, though, I aren't an expert.
Jon