D.C.
Keep the crayons, pencils, pens, and markers out of reach as much as possible. Whenever my kids would write on the walls, the consequence was that they had to clean the wall with me, while being reminded the whole time that writing had to be done on paper or coloring books, and that it was hard work for mommy every time she had to clean up the mess. Time outs do not work as it seems they are always the consequence for every mis-behavior and kids don't really understand what they did wrong, just that their in trouble for making mom or dad mad. Tell him ahead of time that writing on the walls is wrong, and that every time he does it, he will have to help you clean it up, and stick to the same punishment every time. He'll get the message. It did for me.