Camera App keeps on updating Google made their camera app update available on the play store. This update does replace the system app.

BUT I do not want that update, since I use the countdown-timer, wich is not available after the update.

So I went into app-mgmt, deinstalled the update, unchecked notify-updates.

But - it still keeps updating that app from time to time.

I even disabled the general option in play to notify me when updates are available. That disabled updates for ALL apps.

After manually updating all other apps - guess what was updated again automatically?

Yes - camera.

So how can I permanently disable that specific update?
The notify when updates are available option has no effect on auto updating apps. You need to disable "Auto Update Apps" right below that option. Also, in the Play Store, go to the camera app listing and tap the menu button (either physical menu key if equipped or the 3-dot menu in the top right) and make sure "Auto Update" is not checked for the camera app. This should prevent it from updating. Keep in mind this update will still appear in your list of app updates in the Play Store, so you will be unable to use the "Update All" function since this will include the camera app in the apps to be updated.