A: First, note that Ensign Smith is the Star Trek: Original Series Fluxx, version of the Expendable Crewman, and Tasha Yar is the Star Trek: TNG Fluxx equivalent. They all work the same in terms of having the “power” of impulsive self-detrimental tendencies.
Also note that Star Fluxx has the Laser Pistol, while the Star Trek Fluxxen have the Phaser, and that they work ever so slightly differently. The Laser Pistol can only target a Keeper if afflicted by an Attaching Creeper, and the Phaser can only target a Creeper, so you can only target the Expendable Crewman (or analogue) directly if they have a Creeper attached to them.
However… since the Expendable Crewman is known to jump in the way of a laser beam endangering any other Keeper, you could pursue an insidious plan whereby you target some Creeper-attached-to-a-different-Keeper belonging to the player who has the Expendable Crewman. The Expendable Crewman will then leap into the fray to protect the other Keeper which has a Creeper attached, unwittingly becoming the victim themselves.
While the Phaser can target Creepers that don’t attach, note that no Expendable Crewman worth their salt would take action to protect a stand-alone Creeper, so you’ll still be limited to attacking Keeper-Creeper combos if you’re trying to dispose of an Expendable Crewman or analogue.
So, while you can’t necessarily target the Expendable Crewman (or analogues) outright, it’s absolutely still possible to do it on purpose within these parameters.