There’s a queue of dwellers waiting at your vault door and maybe you want to get rid of them. It’s actually quite simple, first you let them into your vault and then you evict them.
To evict a dweller, drag them outside your vault as you would when sending them to explore, but this time click the bottom right red button, the game will ask if you want to evict them, confirm ok, then out they go.
Another option is to let them in and then send them into the wastelands, and eventually they will die. While it maybe a sad fate, if they get good loot you could always revive them and recall.
You can leave dwellers waiting in the queue, but they will wait forever. A long queue will create an problem when recuse dwellers or legendary dwellers cards show up, as you can’t access the back of the queue. For this reason you should reduce the queue at regular intervals.
What if vault is full with dwellers waiting in line.
Once you hit 200 dwellers you can no longer accept new recruits. There’s not much you can do, without the ability to let them in, they’re stuck in the line. One way to handle them it to have a vault of 199, and then you have a way to manage them.