ah... saying BX slider through me off.... that isn't BX Slider. That is different 3rd party gallery. you see those 2 squares above the slider? Click on the right one. I'd have to mess around with the CSS/HTML/JS a bit, but I think it can be set to default to the thumbs off (right square) and then they could turn the thumbs on.
Other than that, just hide it via Extra.css: