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
To toggle between Paint Select and Paint modes in the following steps, use the hotkeys + > and + < to change modes to the right or left.
As you continue painting weights, only those vertices you selected can receive weight values, and all other vertices are masked.
You can also use the Hold buttons to quickly set specific influences you want to work with. For example, to hold the weights for all but three influences quickly, select the three influences, click Invert Selection to select all other influences, then click the Hold button . The weights for all selected influences are held, letting you paint weights for only three influences.