You can create a mask on the mesh that lets you paint specific vertices only, blocking all other vertices from receiving weights.
To create a mask while painting skin weights
As you continue painting weights, only those vertices you selected can receive weight values, and all other vertices are masked.
to select all other influences, then click the lock button
on any of the selected influences. The weights for all selected influences are locked, letting you paint weights for the
original few influences. See also Lock smooth skin weights.
Except where otherwise noted, this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License