Hide UISlider thumb image

Much simpler:

Objc

[sli setThumbImage:[[[UIImage alloc] init] autorelease] forState:UIControlStateNormal];

Swift version

sli.setThumbImage(UIImage(), for: .normal)

The easiest way is simply setting the Thumb Tint colour to Clear on the Interface Builder - like so...

enter image description here

Voila!