Preventing the built-in flash from firing when using a Nikon SB-900 flash

When using the Nikon SB-900 (and the SB-600 and SB-800 as well btw.) as a remote flash and the camera as the commander, the built-in flash always fires as well even if it is explicitly disabled in the e3 menu. Even though those timing flashes are of lower intensity, they can be seen clearly if …