They are trespassing. They do not have a constitutional right to trespass and can be removed.
Actually, you have made my point better than I could. The hypothetical devil worshippers/Obama campaigners on your lawn, while engaging in Constitutionally protected activities do not have the property owner's permission to practice their Constitutionally protected activities on his property, and therefore are trespassing. Similarly, when a property/business owner says effectively, "Persons carrying firearms are not allowed on this property," they are denying permission to such persons, and persons carrying firearms on such property are trespassing, although carrying a firearm is, like devil worship, or campaigning for Obama, a Constitionally protected activity.