Removing ink stains from walls isn't easy. Use the wrong cleanser and you risk spreading thinned ink even further or removing a layer of paint along with the ink. Though it may be time-consuming, you can remove ink with rubbing alcohol. Careful blotting with the right materials pulls the ink from the wall bit by bit, leaving the surface clean with no sign of the ink having been there.
Pour a small amount of rubbing alcohol onto a cotton ball to dampen it to a point that's slightly short of saturation.
Place the cotton ball onto the pen stain on the wall. Use a patting motion with the ball to blot the ink from the wall. Examine the ball after about 10 seconds to make sure the alcohol is thinning the ink and pulling it away from the wall's surface.
Change cotton balls when you've covered the one you're using with ink to avoid spreading thinned ink over the wall surface. Continue to blot at the ink, removing a small amount with each ball until all that's left is a faded tint on the surface of the wall.
Switch to a clean white cloth. Dampen the cloth with the rubbing alcohol and rub at the tinted wall surface with the cloth to remove the last of the ink tint. Switch cloths if necessary as each becomes soiled to avoid spreading the thinned ink.
Dry the wall with a clean cloth after completing the removal.