This could be done by applying the invisibility potion effect to be done simply.

Just give the spectator the invisible effect...this isn't a bug

-Daniel and Eddie

(I'm daniel btw) this could be achieved via potion effect by using addPotionEffect(new PotionEffect(Potion.invisibility.id, 50, 1)); . Please note that 50 is the duration and 1 is the amplifier of the effect and you should experiment with those.