I plan on asking my gf to stay hanging with a certain guy.
Why do you want her to stop hanging around him?
I am afraid that he is going to take my gf from me.
Has she given you any reason to think she's interested in him? You should talk to her about it; there are likely innocent reasons that they are hanging out. It's better to try to trust her before being suspicious.
She's a woman, you can't trust them.
While in a relationship you really need to be able to trust your partner. If you find that you are putting your partner on a tight leash than you partner will most likely want to find someone who gives her more freedom. All you can do is focus on being the best boyfriend you can be.