Well, if your space station is a military-type installation, or built anywhere near those standards, I can imagine that they would invest in bulkheads similar to what you'd see on a modern naval ship. Right now, the reason we have relatively fragile things in space is the cost of putting materials into orbit. If space travel has become prominent, then it can be assumed that we overcame that challenge.
So, that being said, if you build a space craft like a modern naval ship...the walls are tough enough to shrug off small arms fire. Low-velocity hollow rounds would probably be the safest...enough punch to go through a squishy human, but a steel wall would stop it cold, with little more than a dent to tell of the impact. It makes sense that a craft built with the potential of being boarded is going to be built in such a way that it can survive the first person to fire a gun onboard. Otherwise, boarders will hop aboard, breach your hull, then just sit back and wait.
Barring the idea that someone would build a space station such that a kid with a .22 wouldn't be able to breach the hull, or considering your specific scenario here are some other options that currently exist.
Sonic weaponry (you can use the right frequencies to reduce humans to nauseous puking messes on the floor), Infrared weaponry (so called 'heat rays' inflict debilitating pain on organics, but do practically nothing to anything non-organic), anything used for riot control (rubber bullets, beanbags, stingball grenades, gas, etc.).
Flashbang + melee is always good too, that is a very common means for close quarters combat, and is probably what would be used in a situation like the ISS. Perhaps not with your standard flashbang grenade, but the idea is sound: blind/disorient opponents, swarm in and take them down without firing a shot. And yes, this works in military applications as well; I have first hand accounts of someone who was onboard a ship when, during a training exercise, it was boarded and taken over without the boarding team firing a single 'shot'.