Try this, first you finish up showing whatever ad you want and finally call finish, this will close the application. This code willl take care of the back key exit scenario.

@Override
public void onBackPressed() {
    //Do your stuff
    finish();
    return;
} 


Show an ad (revmob) on app exit
My system is encrypted using Full Disk Encryption, i.e. everything except /boot is encrypted using dmcrypt/luks. I am concerned about Cold Boot Attacks.

Prior work:


https://tails.boum.org/contribute/design/memory_erasure/
http://tails.boum.org/forum/Ram_Wipe_Script/
http://dee.su/liberte-security
http://forum.dee.su/topic/stand-alone-implementation-of-your-ram-wipe-scripts


Can you please provide instructions on how to wipe the RAM once Ubuntu is shutdown/restarted?

Thanks for your efforts!
