End-of-rope POA nightmare. What can be done?

Who are the professionals who help with horrid family clashes?

I am POA and HCPOA for my dad in America, and he has 3 adult children from an earlier marriage, who live in another country, but who keep trying to grasp for control over his life in horrible, often illegal, ways.

The problem is that because he has dementia he cannot or will not accept their bad behavior and only sees them as loving children, which puts me in the awful unwanted position of having to protect him from their antics and appear to be some evil cop. I DIDN'T SIGN UP FOR THIS! I am happy to handle my dad's life and care, hard as it is, but I WANT NOTHING TO DO WITH HIS OTHER FAMILY.

They are soul-sucking, joy-destroying, miserable people. What can be done?

Last week, one was visiting and locked out all his caregivers (whom she was told needed to be there even when she is there, to keep him on his routine), and hired an unvetted "doctor" to come in and try to disprove my dad's dementia. (even though earlier this year he went through a full day of testing, MRI, etc. and the results were moderate to severe dementia--this child had seen the results).

I was not physically present, so I called, as POA, to speak to this person in my dad's house. The "doctor" hung up the phone on me and physically pushed the caregiver out of the room and shut the door. At that point I called the police, we presented my POA papers and both the doctor and adult child were asked to leave the house. At the police and my attorney's advice, I let 1/2 sib know they were no longer welcome to stay in the house (this is just one of many such behind-the-back incidents that have occurred in recent months). The three adult kids keep reminding my father over and over of the incident and upsetting him terribly. Otherwise, he easily forgets it and is ok.

Who can I speak to for help? This situation has been going on over a year and they are relentless. Looking for advice. I could block them, but he doesn't want that and does not understand the damage they are doing. I don't want to be the person that blocks a parent from seeing their children. I don't want to be in the position, period!