Turn off the power to the room in which the fan is located by moving the corresponding circuit breaker to the "off" position.
Detach the glass dome or shades from the lighting fixture and remove the light bulbs. Use the ladder if you cannot reach the light by standing on the floor.
Locate the point at which the lighting fixture attaches to the fan; a lighting fixture attaches either directly to the fan or to the circuit box in the ceiling.
Unscrew the lighting fixture from the fan or the circuit box. Be sure to support the weight of the lighting fixture as you remove the screws; the wires running from the lighting fixture to the fan are not strong enough to hold the fixture up by themselves. Lower the lighting fixture several inches below the fan to expose the wiring.
Locate the wires running from the lighting fixture to the fan and remove the plastic caps from those wires. Disconnect the lighting fixture's wires from the fan's wires and replace the caps over the bare ends of the now unused wires hanging down from the fan.
Install a cover plate over the exposed wires on the fan. Most fans come with a cover plate to account for the possibility that someone might remove the lighting fixture. If you do not have the cover plate for your fan, buy one from the fan's manufacturer or a major hardware retailer.