DIAGNOSIS Faulty or stuck 5-way joystick controller. Debris or a manufacturing defect causes false inputs.
FIX Power the monitor off. Use a can of compressed air. Blast air directly into the gap around the joystick nub. Move the joystick in all directions while spraying. This dislodges dust. Power back on.
IF THAT DIDN'T WORK The joystick module is likely defective. You can temporarily control the OSD using the Samsung DisplayWidget software on your PC. For a permanent fix, the entire rear control board must be replaced by a technician.
HEADS UP Do not use liquids to clean the joystick. Moisture inside can short the board. If under warranty, Samsung will cover this repair.