This question applies to my nexus 10 also... and my rooted s3... being that you use the volume button when taking a screenshot, the volume bar always appears in the shot. how can you make it go away?
You must be doing it wrong (ie: hitting the volume control prior to taking the screenshot). Hold the power button and THEN press VOLUME DOWN (ie: minus).
Also, there are apps that you can download that will let you take a screenshot by shaking the device. You open the app, enable the shake, then get to whatever you want a pic of and shake shake shake. I just got used to the button pressing, but there are other alternatives.
What I do is hit the power button first to turn off/lock the screen then press the Power / volume down buttons at the same time... Never get the volume bar in my screen shots doing it that way.
Actually... Nevermind on my last post... That's what I do to reboot my N7 and jot taking a screenshot.... Again... Sorry. I'm getting ready to go on vacation and not thinking clearly... LOL!