Press and hold on the notification.  This will give you a few options.  I forget what one i clicked on but it brought up the options for the message+ app.  Scroll down and turn off  background me... Ver más...
Press and hold on the notification.  This will give you a few options.  I forget what one i clicked on but it brought up the options for the message+ app.  Scroll down and turn off  background message sync.    That did it for me.  I have the Pixel2.