With ISO paper sizes, the loophole is easy: Buy A3 paper, fold it in half and cut along the fold for 2 pieces of A4. A3 banned? Fold A2 in half, cut, now you have 2 pieces of A3. Another loophole? Most printed paper is only one sided! Turn it over and now you have a blank page!

But seriously, Chinese government, you're making yourself look idiotic. If your own people can make you so afraid of a piece of blank paper that you need to try to ban the sale of it, you've lost.