How to stop someone from downloading Roku camera app and having their own account and password and being able to have access to my camera without my knowledge
What makes you think that's possible? It shouldn't be, so if you have a story to tell you should share it.
because your camera is paired to your account during setup.
Those who want to share cameras with multiple Roku accounts, can't. So, it won't work without your okay either.
It is possible because it just happened to me. I’m out of town and got a notice of something I did not recognize using my wife when I paused it I lost my camera all together.