Can a hospice organization legally dump a patient without notice and without cause?

There is a greivance procedure that can be filed on the phone or in writing. The greivance policy must be given to the patient or family on admission (a copy) and posted somewhere in the facility so it is visible to everyone.
Find out the regulatory agency who gives the hospice their Medicare certification and make a complaint. Get the person's name who you spoke with. In my experience if someone reports abuse, the agency responds and will send in a team of surveyors to investigate the complaint and usually pretty quickly- like less than a week.
Or go directly to the CMS website & search how to file a complaint. Find out who the regulatory body is for nursing licenses and call them to document a complaint as well.
What's been determined for your loved one? Have they resumed care for them?
I am sorry this has happened to you and your LO. I don't think they can stop providing care and need to have a darn good reason to discharge her and usually give them 30 days notice,.
You have rights here. Don't give up.

These things happen because NO ONE enforces these laws or rules or regulations, least of all, Medicare and Medicaid. One files a complaint with Medicare AND the local police and nothing happens. The RN had fled before the police got there and was not pursued. The police did not even allow me to make a written report, and later I learned that no report is even on the record. It's actually worse than I described due to a Hawaii State Agency, Adult Protective Services, which came to my home 3 times. The second time I told them not to come back without a search warrant. They came back without a warrant anyway, but with the police, also without a warrant, who broke into my home to allow the two people inside. I would file a law suit, but can't afford it, and there is no assurance the courts would be fair in any case. The police and court situation here is similar to that of Nazi Germany. Until we become millionaires, we just have to take it as is, or move to another state or country.

Are you saying they arrested the RN at your home or a freestanding hospice center?
What kind of abuse and for how long as this been occurring?
Medicare has guidelines for discharging any patient whether it be from hospice or other facilities and those guidelines must be followed before discharge. Usually they advocate more for the patient than the facility. Did you know this was going to happen with the RN? Who did you tell about the abuse?
Also usually before someone is arrested an investigation is done so the police have enough evidence to prosecute the abuser. They can't  just arrest a person without proof. Especially health care providers so they can't counter sue for false accusations.

I'm curious what kind of physical abuse you and your poor wife were being subjected to? Are you pressing charges?

Is there another hospice organization you can call?

You were physically absurd by an RN. You called the police who removed the RN and now the hospice facility is refusing to take care of your wife while she is dying? Is that what you are saying?

We didn't agree to leave, they just abruptly stopped serving us without notice. We needed what little care and equipment we were getting, as we could not pay for any kind of care. I'm complained to Medicare, but they don't seem to care either.

That sounds like a horrible experience. I'm sorry to hear that such a thing is even possible. Are you saying that now after this happened with the RN that you are being asked to leave?