As GM you can override posts. If someone finds a rare object make it a fake made out of plastic. Someone kills a monster in one hit? Have the thing dodge and counterattack. Make sure you have a firm rule set and even if your roleplay dies don't let people walk all over you. I'd personally rather have a dead roleplay over a roleplay where everyone is doing whatever they want regardless of the story. The troublemakers typically don't come back afterward.