Xbox 360 help!!

Can you not set up passwords on the xbox when it boots up? Pretty sure there is something like that which means he can't use it without knowing the password.

Break his fingers?

Take the power brick with you until he splits the cost. Brothers, yo-- I KNO DAT GAME

(My little brother did exactly this, I then realized what a douche I had been. Sharing is the tits, y'all)

Set up parental control. (I'm sure it has it)

Keep it secret and hook it to your PC monitor. :)

You just have to hide the gamepad.

You can password protect gamertags, but I think that might only be if you try to access xbox live, meaning he'd still be able to play games offline on the console. Don't quote me on that though. There are parental controls though, which you could set up on his gamertag, but that wouldn't stop him just using your gamertag (although if he wanted his own achievements etc he might prefer not to do that). You could restrict his account from playing all the games that you own, or if you wanted to have a little fun you could set it to allow him the minimum amount of playtime per day before he gets kicked, maybe 10mins or something like that. There isn't anything that I'm aware of that could stop him from creating new accounts, but I imagine if you kept deleting them he'd get pretty sick of having to restart all the games he's played.

Keep using steam.

I'm pretty sure you can parental lock the system up. If not at least restrict it to play only kids games without password. Charge him a dollar every time he wants you to enter the password.

You could always just take the power cord with you, or the gamepads.

You can set a timer i'm sure to one of the accounts, if yours is master account his is the limited account or something like that.