I'm going to not let my brother use my computer
Why don't you want him using your computer?
I'm annoyed because he uses my computer often even when I don't want him to.
I understand your frustration, but siblings should share things with each other, it's normal behavior. You should talk to your brother and perhaps set specific times when each of your use the computer, but don't block him from it entirely. You will look possessive and egoistic and he won't share his things with you when you need it.
I'm getting fed up explaining over and over again why he can't have access to my own computer. I'm considering putting a password on it.
I think putting a password on your computer is a great idea. That way he can't use the computer when you don't want him to and your problem is solved.