If you using the white TV button on the remote and leaving the 360 box on the TV might be picking up some HDMI signal from the 360 and turning back on, if you have one power set to 'on' on the 360 you can press the red button to turn them both off, if you only want to turn off the TV and leave the 360 on then if you do have one power set to 'on' try setting it to 'off ' to see if that makes any difference.
As there is a new version of software being rolled out what version is your 360 running ?