It's a pain that an android application is locked up, crash and frozen on your Nexus 7 tablet. You can normally quit it. Instead of restarting your tablet, you can force close frozen apps in a few seconds.
How to Force Quit Frozen App on Nexus 7
* From the Home screen, touch Setting.
* Navigate to Apps, and tap on the app for which you want to quit.
* In the App info screen, you would have noticed options like Clear Data, Force Stop, Uninstall. Simply tap on "Force stop" button.