Yup, we even see that the final setting beautifully corresponds with what the Tech Manual claims - that it takes a minimum of eight blips on the display to engage the "vaporize" setting. Riker seems to go from stun to some kind of heavy stun/mild kill before opting for the surefire kill.
Yuta could clearly have been stopped from completing her dastardly deed by simply firing at her several times with the previous, non-vaporizing setting; the intended victim could have run away and locked the door while Yuta struggled with the stunning beam. But as per the rules of Star Trek, she had already done all that was allowed of her in this world, and outlived her welcome by some margin. It's not as if Starfleet soldiers are under rules not to kill the enemy. Especially one who is by design incapable of ceasing and desisting or dropping weapons!