You can try disabling the app, if it won't close. Go into "Settings" and then "Apps". Locate and then tap that app. Now tap "Disable". This should stop the app from working at all.
You may also, prior to this, want to start up the tablet in "Safe Mode" and see if this app auto turns on in there as well. If it does not, then you may have something on the device that is causing it to get stuck open. You would then have to remove whatever is causing it.