One either has magical blood or one does not. A muggle is a human who doesn't have any, and saying that they can work spells if they practice enough is like saying a human can learn to fly if he practices flapping his arms often enough. It's just impossible.
[Private]
Ladies do not like to read about fearsome demons and gruesome deaths and forbidden magic. It would distress them.
I know. That's what Papa said. We shan't ever try anything like it again, I assure you! I gave my word on it. But it proves my point, because muggles would have even less chance of survival.