So a little backstory before my question:

I am 29 and my girlfriend (gf)is 30. We moved in together about 6 months ago. My gf has 3 children from a previous relationship; 6yr girl, 8yr and 10yr old boys ( the boys have a different father than the girl, girls father is not in the picture). I have a BD that is 11 that lives with me. Her 3 kids are seriously lacking in the discipline department. The 6 year old isn't really my concern, she is loud, commanding and screams and cries often but I can handle her, she likes me and claims to respect me. However, her two boys are another thing alltogether. They scream, curse, look at pornography (which my daughter found and relayed to us) tell both their mother and I NO when we ask them to do things, lie repeatedly and do not care when they are caught. They stomp up and down the stairs when they are punished so loudly that neighbors call the police and complain that their is domestic abuse occurring. When my gf spanks her 10 year old he screams loudly things like "YES GIVE ME MORE. I LOVE PAIN!!!!" or "YOU WANNA HIT ME SOME MORE?? DO IT!!!!" They don't respect the rules of the house. They constantly bicker and scream at each other for the littlest things. Like, literally screaming loudly at each other. Cops have been called for it. Their bio-dad is an aweful human being, he bought them cellphones recently which both their mother and I are opposed to. I told my gf that this was a horrible idea and she said she wouldn't allow it. Well long story short she ended up caving. So NOW when these kids get punished, spanked, whatever. They run upstairs lock themselves in a bathroom and text and call their dad and tell on us. Their dad is horrible. He is in his 30's has no job, pays no child support. Despite my gf following through with her court order to allow the kids over there every other weekend. Their dad texts the boys and tells them im a faggot and other terrible names. He tells them to disrespect me and to act bad. Which the 10 year old is happy to do. lately it has been getting much worse, the Bio-dad wants to come fight me, I am worried about him breaking into my home or damaging my car. My gf and I have taken every electronic away, phones (which I have brought the hammer down and told my gf I will no longer allow in my home and if she doesn't accept that, she can leave. She fully supports my decision.) game systems, youtube. You name it. They are locked out of the wifi. The 10 year old has taken the phone being taken away very badly. Before we took it away this last time, my daughter and her daughter both saw him with it when he returned from their fathers. We insisted he turn it over to us and he told us he didnt have it, that he had left it at his dads. His dad told us it was not at his home either. A few days after that, I waited until the kids went to school and tore up the boys room and found the phone. He had been hiding it under his mattress. I found text messages in his phone from his father basically encouraging him to keep the phone hidden from his mother. (My gf and her baby daddy hate each other). Now the 10 year old has reached the point where when he gets home (mother is at work), he screams at me, says he doesn't have to listen to me, and basically laughs in my face. This morning he finally found out that his phone was missing, I woke up to him pounding on my door and telling me to give his things back that I have no right. I don't mean sadly screaming and pounding. I mean pounding like the police and screaming with anger and violence. I told him if he wasnt out the front door waiting at the bus by the time I got up and dressed (i sleep naked lol)that I would smack him. When I got up he had ran out the door. My daughter is very well behaved, she is an angel. She does do wrong often but she is polite, she says yes ma'am and yes sir. I am concerned for her being around these bad kids. I have reached a point to where I am confronting my gf about these things. She says shes sorry that her kids are so bad, she doesn't know how to control them. She cries and collapses in defeat when they treat her badly. She does spank them but like i said, they don't care. Her kids are big for their age and they are getting to the point to where if they desired, they could hurt her or push her around. I am at my wits end. I have never laid a hand on her boys, but I have no idea what else to do at this point. They don't respect us. They dare me to strike them. They don't think I will do anything. My relationship with my gf is amazing, shes a wonderful woman. She is very worried her kids and her baby daddy are going to be the end of our relationship. It eats away at her and she cries even at work sometimes because she thinks I will leave her. I told her I'm not leaving her, but that eventually things can reach a breaking point where I will have to take my daughter and leave. I don't want this to happen to my gf. We deserve each other.

Here are my concerns:

1) I am worried that nothing will be done, and these kids will get into their teen years and then they will be trying to fight ME. And due to their size it wont be an easy for me.
2) I am worried about their father damaging my property. I can't afford cameras to watch my home.
3) I am also worried about their father trying to fight me. He is also a large guy. I can hold my own, but he is much bigger than me and way out of my weight class. I fully believe that he would kick my door down and attempt something if he chooses to do so.

Please give me any advice you can. I feel like a stranger in my own home.