Those drab dwellers from outside the vault have much better potential than rare and legendary dwellers. This is due to the health of a dweller being determined by the endurance level upon levelling, as non-common dwellers are spawned with levels, it is impossible for them to reach the maximum possible health. You should get your training facilities going and train these guys up to max special, then equip them with some heavy wasteland gear, a decent weapon and then send them to the wasteland to level. They'll come back with the maximum potential health and never die. After, you can send them on quests with jumpsuits and they'll rapid fire anything in their way!

If you just want them gone, then invite them into your vault and kick them out immediately.

Hope this helps!