Slab

Slabs were robotic beings, typically androids, used by individuals of various races (typically criminals) as basic slave drones. Slabs are over six feet tall and tend to travel in pairs. They had a vague form as to not provoke suspicion, but their attitude and internal anatomy were very unusual - they always obeyed the commands of their master, disrespected other beings, did not speak, cough, sneeze, ever be heard breathing, or even seemed to feel pain (as shown when it didn't make a noise when it got killed by the Judoon blaster) and had the same composition all the way through.

For example, the Slabs used by a Plasmavore, Florence Finnegan, on Earth (and, due to the Judoon's intervention, on the Moon as well) looked like motorcycle couriers dressed all in black including the head, body, arms, hands, legs, and feet, but they never delivered anything and were composed entirely of leather, except for their helmets. They could be "killed" by sufficiently high levels of radiation, such as an X-ray with radiation increased by 5000%, or energy weapons such as Judoon blasters (TV: Smith and Jones)

During Smith and Jones one was killed by Martha Jones and the Doctor - the Doctor increased an X-ray's radiation by 5000% with his sonic screwdriver and Martha pressed the button, and the other was killed by a Judoon blaster used by an unnamed Judoon trooper serving under the Judoon Captain.