If your connection with her is strong, you won't have to ask her to tell the other guy to hit the road. He's not the issue you need to be looking at. A woman who is happy with you is generally not going to be entertaining another guy, no matter how much he pesters.
If you feel weak and threatened, then you will act that way with her, and it will become an issue she has to think about and deal with, and this is how you fall into a self fulfilling prophesy.
Operate from a place of strength and positive actions. You will feel more empowered and in control of the situation.