"We can ban the "klan" but not Christianity. "
We don't ban the "klan." It's a perfectly legal organization. There are chapters in many cities, and they often hold little demonstrations.
We CAN ban cross-burning, lynching, and intimidation. We don't ban the KKK.
Maybe we've reached an accord:
"We don't ban bank robbers, but we do make the actual robbing of banks a crime."
And we don't ban Wahhabi mullahs who might preach the killing of infidels, we just ban the preaching itself.
So we're back to "clear and imminent danger".
Now you say, "But that's a violation of the 1st Amendment . . ."
Simple question for you:
Do you agree with sharia, and should it be allowed?