No true Scotsman or appeal to purity is an informal fallacy in which one attempts to protect a universal generalization from counterexamples by changing the definition in an ad hoc fashion to exclude the counterexample.
Buddhism has a lot to teach us, you know. It has been shown to have real psychological and cognitive value. You mean like those Buddhists monks who helped the Burmese Army ethnically cleanse the Rohingya to the point of genocide? Oh, but they weren’t proper Buddhists. – The Four Horsemen, Foreword by Stephen Fry. (I think he’s quoting the above from somewhere, but I’m not sure of the source)