Enforce something close to realism (or realism within the confines of the world that you have created). That is all you can do, really. From that, you have to hope that the people in the RP are respectful enough to not to be god modding dicks. If people do try to present themselves as overpowered, remind them that nobody wants to read about people who win at everything all the time and too much winning will make them boring. Losing creates weight, and risks make it feel as if something is at stake. If Star Wars was about how the rebels build twelve death stars and shot them all at Darth Vader, that movie wouldn't have been popular beyond grindhouse drive-in's.

As for writing the combat itself, the best way is to write quickly and somewhat matter of factly. Think in terms of how somebody actually in combat would experience it. Even if you are not writing from a character's perspective, quick moving writing gives the sense of action so long as it is done right.