One thing I learned with my own son is to just simply say "NO"
I had too many times where I would say "I'm sorry but I just can't help you this time" would lead into a debate.
It would go something like this:
Me: I'm sorry but I just can't help you this time"
Son: but you will help me right? you said not this time so that means you'll help me right???
Me: I can't help you. You need to start taking responsibility for yourself and quit relying on me.
Son: Rely on you, when have you ever been there to help me?
Me: All the times you got arrested I was there for you in court
Son: Ya, whatever, you never bailed me out instead you let me sit in jail.
Me: I'm sorry about that
Son: You owe it to me, I need money
Me: No, I can't help you
Son: You're a

Me: Goodbye
It took time but I finally learned to just say "NO" with conviction and without any apology.
When he would ask me why, I would say "I have made my decision, I said NO and do not need to offer any explanation" The debates stopped.
There is so much power behind that simple little word.