Can't do it. Matter of principle. If I can ban a religion, that means someone can ban mine. Not a good thing.
It's also against the law.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances. --Amendment 1, US Constitution
Should cults be illegal in this country? Whatever happened to freedom of thought? Where would all the Hari Krishna's go? The Sierra Club?