This is really pretty simple. The states just have to say "we are not following that law.", which will have a federal response of, "Don't follow this law, and we will withhold funding for 'x'.". The states should then say to the Feds, "Fine, you withhold the funding ,and we withhold the federal income tax of all of our citizens. ". What most people forget is the tenth amendment. The Feds are continually overstepping its bounds from a Constitutional point of view. Feds have no authority to dictate to the states what it can or cannot do , unless it is SPECIFICALLY stated in the Constitution. The power of the states lies in their ability to withhold money to the Feds.