I personally think this is a great idea, as these people are advocating against the kind of religious tolerance and inclusivity that our Founding Fathers fought so hard to obtain. By all means, they should leave and form their own nation. Just as long as they don't deceive others that their version of the United States is the real United States.
And no, let's not have a second Civil War. Let them go, freely, with our best wishes. Take away their U.S. citizenship, their voting rights, and their right to move freely within our country... and let them go. They'll be doing us both a favor.
No hard feelings, and good riddance.