The other method, which is the one I always use is to trust your players to do it right.
This has already been mentioned a couple of times. You put a statement in the opening post saying something like, "There will be player versus non-player character interaction in this game. Players are expected to interact with NPCs realistically".
Then players know what to expect. When a player comes up against a NPC, if they god-mode and auto-hit, you PM them and tell them to change their post. If they refuse, you tell them in the OOC thread to do it and point out why. If they still refuse, you tell them they are out of the game!
You, as GM have the right to do that. You are controlling the NPCs. The player would be told off for god-moding another player character. Well the NPCs are the GM's characters. (Technically if the GM has a "player character", that character is also a NPC, according to some definitions). Simply do not allow a player to god-mode your NPCs.
That said, we are all adults here - or at least, we have all managed to convince the staff that we are over 18. We are here because we want to do text-based role-playing, what some call collaborative writing, in an adult environment. Behaving like a teenager who doesn't like to loose is, in my opinion, grounds for reporting the player to staff, to investigate how old they really are!
So, my best advice in answer to the question is, you don't have any way of stopping a player doing what you said, other thantheir own commitment to write a good story!
A minimum post length rule could be a way of helping prevent such things, in so far as if a player posts that they just kill the bad-guy, that is likely to be a short post. In order to pad the post to the correct length, they will need to add detail of how they killed it, which, if it is detailed and realistic enough, could (not saying it will) qualify the post as being okay.
Even so, the GM can still say "Uh, great post, but you might want to make the fight a little less one-sided? You're just one thief armed with a dagger in leather armour against a squad of battle droids.... Not saying you should loose, necessarily, but you might want to take a hit or two?"