Unfortunately, if the mold is visible on the surface, chances are it is behind the dry wall. If it is only at the bottom you can remove the first two feet of drywall and re rock. However, if the walls are insulated, check to be sure that the insulation has no mold.
Remove drywall and check for source of moisture. Remediate properly and clean studs etc with a mold killing product. Then replace with new drywall. They do make mold resistant drywall for areas prone to moisture.