I've been looking for a freeciv setting that would allow me to choose between razing a city when capturing it and keeping the city. Couldn't find it in the options dialog. I'm asking here to make sure I didn't overlook it.
For now, if I want a city destroyed, I'm using a wave of "Poison city" spy attacks which reduces the population to 1. Then the city gets destroyed when I capture it.
Another way to decrease the population to 1 is using nukes, but this is expensive and has a very negative impact on the ecology, so it doesn't suit me.