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
- Select a smooth bound mesh and open the Paint Skin Weights Tool.
- Set the Mode to Paint Select.
- Paint to select only the vertices you want to keep open for weights, then switch the Mode to Paint.
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As you continue painting weights, only those vertices you selected can receive weight values, and all other vertices are masked.
Tip:
To toggle between Paint Select and Paint modes in the following steps, use the hotkeys 
+ > and + < to change modes to the right or left.
Tip: To quickly lock weights for all but a few influences, select the influences you want, click Invert Selection 
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.
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.