I'm not quite sure what's changed, since I didn't have this problem a half an hour ago with the same bone. Except for my "root" bone, each of my bones deforms the surrounding model correctly according to its weight paint. The root bone, however, while painted, moves or rotates the entire model when posed.
That is the expected behaviour. The root bone is the parent of every bone in your armature, by moving the root bone you are also moving every bone in your armature.
Technically it isn't moving your entire model, it is moving every armature bone that in turn moves each corresponding part of the model. This is like moving the upper arm bone and having the lower arm and hand follow along.
Often the root bone is placed at the bottom of the armature and is only moved to position the entire model. You may want to add another bone to be the root and set it as the parent to your lower spine bone.