You phrased your make-believe scenario backwards. The court cannot claim that gun ownership or media outlets are unconstitutional. All they can do is judge laws against the supreme law of the land, so a law would have to be written which claims "All people shall be obligated to turn in all weapons by April 15th" or "All private media outlets will be shut down effective immediately" or whatever. Then someone brings the case to court. If they lose, they appeal. They can appeal all the way to the SC. If they lose at the SC, "then what?" you ask. Then if people want the law reversed they elect those who will represent their interests in the matter and change the law, or the Constitution, or the ideology of the court. It's truly the worst form of governance, except for all the others.