Sorry to hear about the walls. Here is what I did when my sons were very little. I put the crayons away, and would get them out about once a day, and sat and colored with them in a coloring book.
When we were done coloring the crayons were put away. I never let them have the crayons unsupervised until I was very sure that they understood what they could and could not color. It was also some great bonding time.