It will take a while, but go into Settings/apps/app mgr/Download

Bring up each one and remove the check mark for Notifications.

Then once all of those are done, you need to do the same for the other apps that came loaded on the phone.
Settings/apps/app mgr/ALL
In your case, just leave the one message app with a check mark. Obviously you can skip the apps you recognise as having already been done.

I recommended you do the Downloads first because the app mgr defaults back to the front of the list when you remove the check mark and the Back up.... sigh!



The ....../ALL/ list is extremely long, as it contains all of the Android system files.