You need to get them both to leave.
You will never achieve just getting her to leave - and why do you want someone with violent outbursts under your roof?
The behaviour you're putting up with is gob-smackingly outrageous.
Set a timeline - like 6 weeks?
Tell them they are both leaving and give them each a letter explaining the date they must leave by (keep a copy to show authorites) and also the rules they must abide by before they leave.
- No violence
- No verbal abuse
- No shouting
- Clean up after themselves
- Any breakages or damage must be paid for
- Return their keys and knock to gain entry
Advise them to go to Citizen's Advice Bureau or Benefits office for advice.
- Stop cooking for both of them.
- Do not do any washing for both of them.
- Do not buy anything for them.
Contact the local police and explain the date they will be leaving and ask if special constables can be present when you give your son and his girlfriend notice to leave - stating a possible violent reaction.
If they react badly, the police will also probably decide to be present on eviction day.
Contact the girl's parents and inform them their daughter and your son are being evicted on this date.
If you are renting/ Council accommodation - also let your landlord know. They will have a vested interest in the state and care of the property.
Change the locks 4 weeks after informing them of the eviction date. (Or before then if they refuse to hand over keys). Have only one key and keep it on you at all times. so it can't be copied. If they need to come in, they must ring the doorbell. If they break the door, call the police
Your son might become angry - hence the police (or a neighbour) being present when you hand them their letters telling them to leave.
Trust me the situation will only get worse, if they are verbally abusing you now.
Get them both out now. Have space from each other for the next 6 months.
Cut the apron strings with your son.
They won't be going anywhere better than where they are now.
Do not ever invite them back into your home to stay overnight - even if she gets pregnant. You're too useful as a babysitter for them not to need you in the future.
And yes, they know how badly they are treating you. And right now you are giving them permission to be worst versions of themselves.