After you pressed the "Off" button, does the LCD still displaying the same thing (still showing the last Activity mode)? If so, then you have a faulty button. Try pressing it harder. Else, check the device power configuration (under Device > More Options > Adjust Power Settings) and make sure you did not select the option "I want to leave this device on all the time". Also note the command used to turn off and verify it in device mode. Note: You can't configure the "Off" button directly.