"No True Scotsman, or appeal to purity, is an informal fallacy in which one attempts to protect their universal generalization from a falsifying counterexample by excluding the counterexample improperly."

Person A: "No Scotsman puts sugar on his porridge."
Person B: "But my uncle Angus is a Scotsman and he puts sugar on his porridge."
Person A: "But no true Scotsman puts sugar on his porridge."

https://en.wikipedia.org/wiki/No_true_Scotsman