You can't turn off the fans unless the card actually supports semi passive fan operation. If it does, it should do that automatically when temperature is below a certain threshold (usually around 60C).
Try lowering fan speeds with Afterburner. If 42% is the minimum you can set it to, it may still be possible to mod the fan profile in the graphics card's BIOS to allow lower than 42% fan speeds. But the easiest thing would be to just return/sell the card and buy another with a semi passive cooler.