I think that the only way maybe to stop the Galaxy messaging is to stop it accessing the O2 message centre. With the Galaxy - Open Messaging and then using the bottom left button will bring up a window with settings. In settings you will see message centre. If you delete the number then the Galaxy will not know how to retrieve messages or send them. The drawback is that you will not receive any alerts from O2.
If you don't like notifications you can set them to be silent.
The problem here is that probably you are in a minority of 1 regarding turning off messaging.