It seems like the only way to get rid of that is to adjust what kind of editing is applicable to the layers in the map. If even one of them has "Add" editing, then you will see the + sign.
If all they need to do is "Update" and/or "Delete", just uncheck the "Add" editing from the service and the + sign will go away.....but you need to do this for ALL layers that are editable in the web map.