I'm not going give her my check
Is there a reason behind it?
Yes, my ex-girlfriend and I borrowed some money from her friend and we never paid her.
You need to pay her back what you owe. You can feel upset about it but that's the right thing to do.
I never considered paying her back seriously. If she wants to play hardball, I'm not afraid to show her who's boss and that I've got some connections.
You're going down the wrong path here. It's unethical to refuse to repay a loan. Threatening people is even worse, and could result in a lawsuit or even criminal charges.