My Brother steals my things, how can i stop him?

My 16 year old stepbrother with down - syndrome (A disease which gives you an inability to read write or speak properly - look it up) Constantly steals my stuff. He likes me but we fight all the time. He always goes into my room takes money and cuts my clothing up. I am getting constant lectures from my stepmother about how to deal with it and i feel like i am at fault in the situation. I have talked to my dad but he basically says get over it. I really want a lock on my door but Dad thinks that is a stupid idea. My step - mum pities my stepbrother because of his disease and no one will listen to me except my mum, who can't really do anything to help because she has talked to my Dad and he says he agrees but won't do anything. Please help.