What the Fallacy?!
No True Scotsman
The Fallacy
Making what could be called an appeal to purity as a way to dismiss relevant criticisms or flaws in an argument.
Why it's wrong
In this form of faulty reasoning one's belief is rendered unfalsifiable because no matter how compelling the evidence is, one simply shifts the goalposts so that it wouldn't apply to a supposedly 'true' example.
Example
When told that a peace-loving member of the club had started a fight, the club president insisted that no *true* member of the Peace Club would ever throw a punch, effectively expelling him by definition.