Do you have a budding artist who sometimes uses the walls for his creativity? And, did he use markers?

Cleaning marker off walls is easy with toothpaste. Rub the toothpaste (just the plain kind, not gel) into the marker then scrub off with a damp cloth. This also works on wood (rub in the direction of the wood grain).

If the toothpaste does not work, try spray-on sunscreen. I have not tried this one personally, but I’ve read many places where people say it has worked when nothing else did.