The pilot reported that shortly after liftoff from runway 26, the control stick became difficult to move. He decided to continue the air tow and release at 200 feet AGL. After release, the pilot turned the glider to return to the runway to land. He was still having difficulty controlling the glider and felt that the passenger, who was in the front seat, might have been holding the forward control stick. The pilot instructed the passenger to release the stick. Control was momentarily regained then lost again. The pilot could not maintain control of the glider and it collided with runway 08. An examination of the wreckage revealed no problems with the glider's control mechanisms. An examination of another Blanik L-13 indicated that very little force is needed on the forward control stick to render the rear stick ineffective. The flight was the passenger's first in small aircraft and was part of therapy being used to treat the passenger for a mild organic brain syndrome.