Drawing On The Walls... HELP !

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My 15month old son is drawing on the walls. Fortunately it's with chalk, so it's easy to clean off. However that's not the point. I don't want him to think this is allowed behavior. I have a chalk board for him that his father hand made special, he uses it put then wonders off to ALL the walls in the house ! I've explained to him that the chalk board and outside on the ground is the ONLY place chalk and drawing is allowed .... still he scribbles away on the walls. I was cleaning in the next room and I come in to check on him and legit my dining room is covered in colorful scribbles 😣. Idk how to stop it other than take the chalk away. But he somehow finds it smh ! So ladies , any ideas on how to get this habit broken before it starts to get out of control. My fear is that he will start using other things to draw on the walls with 😵. Any ideas will help ! Thank you !

I took the picture as I was explaining to him this is not where he draws .... so I was not encouraging his behavior as the picture makes it seem like I was.