Poster putty stains can be removed from walls by rubbing a fresh piece of putty against the stain in a continuous circular motion. Rubber cement and other adhesives similar to poster putty actually react most strongly to heat and friction generated by movement. This causes the putty or cement to cling to itself, forming a ball that is easy to remove from the wall surface.
In the event that rubber cement does not remove all of it, use steam to loosen the adhesive further using a combination of water and heat. This can be achieved by plugging a clothes iron into an outlet, and then rest the iron with the hot side facing the adhesive stain. Allow the heat and steam to permeate the area for 5 minutes. Once the area is sufficiently warm, use a hot, damp paper towel to rub the area in a circular motion, similarly to the first technique. A combination of heat, dampness and friction should remove any remnants of the poster putty stain from the wall.
If the adhesive does not come off right away using the heat technique, then try to repeat it using a longer period of time for the heat application.