Just because you turn off notifications of a sync in the background doesn't mean you have stopped the sync in the background! The reason it is on in the background in the first place is because you message+ across a few devices. If you don't... As I don't... Here's how I actually turned it off rather than silencing the notification:

Open message+>> click the menu>>settings >>account>> manage devices>>

There are two sections under manage devices. 1 will be primary device... Which should be the phone you are using. 2 should be other connected devices. If you are only using message on your phone there shouldn't be any other connected devices. If there are press disconnect. That will completely disable the sync since there are no other devices to sync with.