In the space-fairing societies that consider themselves civilized, certain rules have developed for occasions when fighting takes place aboard a ship or space station.  These rules derive from the hard realities of life in space.  Spaceship hulls tend to be thin to conserve weight, and the survival of the crew depends on certain life-support systems working.  The Empire of 1,000,000 Suns developed essentially the same set of rules that developed in Human-controlled space for these very reasons.  These rules only apply to combat on board a ship or station, naval actions between ships have their own rules of engagement.

Combat aboard a ship or station is hand-to-hand.

A variety of exotic melee weapons may be employed, including plasma-swords,  nano-edge graphene blades, stun batons, etc.  Ranged weapons are actually illegal for shipboard use in most cultures because a stray shot could pierce the hull, or damage life support systems.  The same goes for explosives and gasses.

Deliberately damaging or targeting life support systems is forbidden.

In a boarding action this benefits both the attacker and the defender.  Whoever wins, at least the air supply won't run out.

Survivors on the losing side are to be returned to a habitable location.

Depriving the losers of a battle of life-support is not allowed.  Forcibly removing a sentient life-form out of an airlock is considered murder.  Survivors of a battle are to be returned to an inhabited station or planet, where authorities can deal with them.  Most ships include a brig, where prisoners can be kept.  

In extreme cases, the most dangerous captives are marooned in an escape pod, left to fend for themselves in the void of space with minimal supplies.  This exonerates the captor while ensuring the safety of the ship.

There are some entities that do not abide these rules, the Engine of Negation (the extremist branch of the Cult of Troniac bent on the elimination of organic life) is an example.  For this they are considered barbaric, and dishonorable, and typically not afforded the protections set forth in these rules.