OP, I think you need to take a mental step backwards and look at the situation from a different perspective.
When your son was at home with the GF, was that what you wanted for him? As I recall he wouldnt go to college because she didnt want him to, he wouldnt get a job and neither would she, you were being pushed around and generally made to feel uncomfortable in your own home.
Is this how you want your son to live his life - not working, being poorly influenced by the GF, not studying, abusing you physically and thinking its OK.....
If it were my son, this is not the life I would want for him. Allowing him (and of course her) back into you home will mean more of the same with your son never actually achieving what you had hoped for him.
Now imagine a different scenario. They have to cope by themselves. SUddenly there is no automatic food delivery, they have to live in a cruddy bedsit or a B&B, where its great for the first week but then they start to think....how will we pay for XYZ? The laundry doesnt get done, theres no money for beer, - sooner or later they get a job. He goes to college, they start to be fully fledged adults who live responsibly.
Its called tough love, and I think you have to do it - not for yourself, but for your son, who will end up a loser if he is allowed to come back into the lap of luxury with no responsibilities and a sponger girlfriend in tow.
Is this the life you want for him?