When I use whiteboard markers on my board it moves the cursor on my computer.

Make sure that the USB cable going from the board to the computer is unplugged. Also, if your board is not powered by USB, turn the power off at the wall.

Reason:
Because the projector is separate from the board, when you turn off the projector and start writing you still have control over your computer. Even though you may be writing on a blank board, if it's plugged into your computer it will pick up where the marker touches and respond by moving your cursor.