Why do my citizens move in when I capture someone else's planet?

The main problem is that I play as a Swarm Devourer and already have a lot of planets, but I don't need any more. I've disabled land reclamation from captured races, but 100 of my citizens still arrive, forcing me to spend 200 workers. I can't destroy the planet, so there's no profit, and it's expensive because my fleet and army are powerful and can capture everything much faster, simply committing genocide on the captured planets.