If my 17 yr old son threatened to hit me, he would be out the door! I would lock it behind him.
That is just so wrong on so many levels, and how disrespectful!
I imagine that you are worried sick about him being with this girl, drugs, etc. but sounds like he is not going to listen to anyone, so therefore he has to live it, and hopefully grow up to see the light.
I hope this girl doesn't do anything crazy like get pregnant, would be a worry of mine.
All I can suggest to you, is let him go. He already knows that you do not accept his GF on all levels, plus the road he is going down is not a good one.
When he gets hungry enough, he will be back. I'm sorry. I could not imagine.