In this case, the specific bones affecting the vertices needs to be found. Once that process is complete the bone weight can be removed completely by removing the vertex from the bone's Vertex Group or the bone weight painting for the unneeded bones reduced to zero.
With the low poly density in this case
- Select the mesh Object.
- Enter Edit Mode.
- Select ONE vertex. (One in the Thigh to Calf area for the above image.)
- Open the Properties Menu. (N key by default.)
- Locate the Vertex Groups Pane on the Properties Menu. It can be noted by the listing of any Vertex Groups you have defined and the Normalize and Copy buttons.
All Vertex Groups influencing the selected vertex are listed in this pane and by how much in a 0.0-1.0 scale. It is highly likely that a child bone (Most likely a fingertip in your example rig; as alluded to by Leon Cheung.) is weighted to the vertex. Any weight, even a minute 0.000001 can influence the mesh if no other bones are acting on a vertex.
This process may have to be repeated for multiple vertices. A slightly quicker way to remove vertices from unwanted Vertex Groups can be used if there is a pattern. Once the offending bone is found: select a group of vertices to be removed, open the Properties Window (found between Logic Editor and Outliner) / Object Data Panel (found between Object Modifiers and Material) / Vertex Groups Pane, select the "bad bone", click Remove.
The most likely suspects in your example rig would be Finger_Middle_3.L, Finger_Index_3.L or Finger_Thumb_3.L. All are lower than the other bones and close to the model's robe.