How to prevent MCE remote from accessing XBOX 360

Configure your Xbox 360 console so that you can use a Windows Media Center remote control to control Windows Media Center functions on your console. Here’s how:
Note You can't use this remote to adjust the volume level of the console.

1. On your console, go to settings and select System.
2. Select Console Settings.
3. Scroll down, and select Remote Control.
4. Select XBOX 360 REMOTE (not BOTH REMOTES).
5. Press B on your controller to return to Xbox Home.

• If the console is turned off, press Start on the Windows Media Center remote control to turn it on and to start Windows Media Center.
• If the console is turned on, press Start to go straight to Windows Media Center.

Note The Xbox 360 Universal Media Remote is the best way to access all functions of Windows Media Center and the Xbox 360 console. It includes TV controls and Windows Media Center controls.

http://support.xbox.com/en-US/xbox-360/windows-media-center/windows-media-center-use-remote-control