You got to let someone go.
There must be someone in your vault that you just can't stand. get rid of them.
Quick way is to make an isolated production room, rush till they die from incidents.
Long way, send them out to the wasteland. Maybe they pick up something good and redeem themselves, maybe not and just die.
Only then will you be allowed to move those 10 out of that line.
You can't dismiss them, you can only admit them (or not) into the vault. There's only so much room in the vault.
You have to make the tough choice: Who are you going to off to make room in the vault?
This is the most sensible advice I've read.
The ones that die too soon have low health (meaning their END was raised after they reached higher levels) so, kill them off and replace them with the ones that are lined up outside the vault. Start training their END immediately.
After you cleared your line, you might want to think about dropping to 198 or so dwellers. Just in case you get a Legendary Dweller from a Lunch Box.
I keep my vault at 198. All are maxed SPECIAL and most are level 50. If I get a non-Legendary Dweller from a lunch box, I strip them down and send them out to die... keeping my vault at 198. If I get a Legendary... I pick some unlucky soul and send them out to die.