So basically you object to your son's guest. That is what she is! She has a permanent elsewhere. If you have a written policy on tenants having a guest, than you might have grounds for enforcement. That means giving your son's notice to cure the violation.
Filing an eviction against the girlfriend could cause you legal repercussions as she is not a tenant, but a guest by law. Any action you take must be addressed to both your son's, as they will be the legal responsible party for evicting.
As to the car, have your son's take care of that. Do not open yourself up to any legal trouble by going about this the wrong way. As a personal thought, it seems you have a personal grudge against the girlfriend. This has no business in a landlord business as it always cost the landlord in the end more. You need to pick your battles, force your son to forbid his girlfriend and the resulting conflict with your son or let things settle themselves over time?