You must kill them without directly attacking. If they are too close to the village when they die it will postpone breeding for a short while, so it's best to move them away from the village.

Are nitwit villagers useful?

Nitwits are essentially a mob that is most useless of the mobs. They cannot be assigned to a job as they don't have a profession. You can think of these nitwits as the villagers who are just there and without a purpose.

Can a nitwit villager get a job?

Nitwits can not gain employment. Unemployed villagers will claim new job blocks and become that profession. While villagers are unemployed they will not trade and will wander a lot looking for a job block, so they act a little bit like a nitwit, but are not nitwits.