Did you actually say it is sinful to manipulate someone to keep them from sinning?
What exactly are we talking about here? One person, with sin in their own heart that needs to be dealt with, taking the time and effort to insure that another person's behavior is not sinful.
No one can make certain that another person lives a perfect, sin-free life. We cannot even make sure that we live such a life.
So ultimately when it comes to "Christian" Game, one person manipulates another person in order to prevent select sinful behavior - in a way that directly, personally benefits the manipulator.
Feel free to read the New Testament and decide for yourselves whether or not such a course of action would be considered righteous.