Is it possible to conquer an enemy system and not settle it?

I want to destroy an AI player that I am at war with but I don't necessarily want all of his systems. Is there a way to just destroy and not settle an enemy system?

Yes.

In the upper tree, on the left edge about 1/3 of the way up, there is a tech called Plasma Cage; it costs 1350 science to research. It grants the ability to build a system "improvement" called the Plasma Raze Grid. This improvement always takes 1 turn to build, no matter how little production a system has (so a just-conquered system with 1 population, negative happiness, and no production can still build it in 1 turn). Once completed, it destroys all improvements in the system, removes all population in the system, and gives you a colony ship.

Yes! There are numerous technologies for ransacking enemy systems as of Disharmony. I believe you're most interested in the special weapons (last) category.

• Vanilla bombardment: captures system. Not what you want if you don't want to own the system (e.x. if you're having happiness trouble).
• Bombs: New to Disharmony, bombs are a targeted weapon to abrade an enemy system into a more manageable state:
  • Anti-population: reduces population of planets
  • Anti-structure: removes orbital structures built in system
• Boats: New to Disharmony, all fighters and bombers can attack a system, even when they're configured for space action. There are higher levels of boats (shielded fighers, heavy bombers, etc) that I haven't been able to try yet.
  • Fighters: anti-boat, meager damage against ships and capture rate on systems
  • Bombers: anti-ship, decent damage against ships and capture rate on systems
• Special weapons: New to Disharmony, there are a few weapons of stupendous, system-cleansing power. These are about a third of the way up the Warfare tree, more towards the edges. One is called Plasma Cage, and is apparently a "system improvement" that razes all population and structure from a system. The other is called Deconstructor Drones, and will convert all system improvements into Dust but leave the population untouched. (Thanks Oblivious Sage!)