Problem with boys such a nasty little idiot?

Hi guys, I'm new here, I'm a 16-year-old boy. I have a huge problem. Here's the thing: my mum's boyfriend and his 10-year-old son have been living with us for two weeks now, and the little guy thinks he's a really cool rapper. I've often hidden his things from him. I'm having a bit of a battle, and yesterday the little idiot made up an excuse and said he had to do something for school on my PC. And now he's installed parental controls on my PC and set it so that I can only go on websites for children, just Kika, Tigerentenclub, and the like. He's also set a time limit of one hour a day. He's also given me a Winnie the Pooh desktop background, and I can't change it. He's put a password on everything, and he's the admin now, and he can't get it off. He just said, "So, the battle is over," and laughed. I told him it was embarrassing, but he just said "bad luck" and laughed. My parents aren't helping me; they just say it's my own fault. What should I do now? How do I get rid of it? Like I said, he's 10. Thanks in advance!