Hello and Welcome to Mrs. Clean's Forum!
Mr. Clean Magic Erasers work great on painted surfaces. A package of 4 can be purchased from a grocery store for about $5. (USD). They really work like magic, and remove even the most stubborn stain safely from painted surfaces. You can read more about them at
http://www.mrclean.ca/en_CA/home.doIf that doesn’t work, tri sodium phosphate, or TSP, should do the trick. It comes in powder form and is available at most hardware stores. You’ll want to wear gloves whenever working with TSP. Follow the directions on the package and dilute with warm water. Wash the wall with this solution and a clean, soft sponge. This should remove the stain without harming the paint on the wall.
As always, be sure to test in an inconspicuous spot before using on a larger area.
Good Luck and let me know how it turns out!
Mrs. Clean