I'm going to tell my girlfriend I don't want a friend to visit her
Can you really do that? Why do you have to stop their friend like that?
I'm sick of my friend, she won't leave us alone
This friend of yours sounds very clingy. You should tell her to back off because it is affecting your mental health. Your friend needs to give you guys space or it will bring you negativity that you don't need in your life.
Nah, I don't want to be nice to her anymore.
I understand that this friend is clingy, and that you need alone time with your girlfriend. She needs to know some boundaries, and it's good you want to try to have her show some space. But how about doing it in a nice way, so she won't be offended?