I'm going to give a girl who was flirting with me a fake number
Why would you give her a fake number?
Because I'm not interested, I have a girlfriend already
It's perfectly fine to not be interested in someone. However, it's best to just be honest with her that you're not interested at the moment.
I don't want to waste my time with this, I'll just give her a fake number and move on.
Ask her for hers instead and tell her you will call her if your current relationship doesn't work out. It's always good to have a back up just in case.