You can make it go away by pressing the 'OK' button on the steering wheel (once it's up), but I don't think you can stop it from coming-up in the first place.
On my 2013 W204, it comes-up shortly after driving off (i.e. when the car goes above ~5mph).
It will then stay for ~30 seconds, unless the OK button on the steering wheel is pressed in which case it will go away immediately.
If a rear seatbelt is fastened or unfastened during the journey, it will come-up again.
What I am missing is the ability to call it up at will... e.g. if I didn't check the rear seatbelts situation in the first 30 seconds of the journey (and yes I know I shouldn't drive off before checking the rear seatbelts)....
My car does not have rear seat occupancy sensors btw, so it can only sense if the seatbelt is fastened or not, but it does not know if the seat is actuality occupied or not.
Ah, so Mercedes obviously changed this slightly, sometime between 2006 and 2013.
In my manual, it clearly states the sign will go off, when the vehicle is over 5 km/h.
Perhaps their research suggested that, people more likely to notice the symbol after they moved off?
I do agree, there should be an ability to recall the status of rear seat belts ... even though I only had passengers in the rear twice, in the last three years.