  We show that the resistivity plateau of SmB$_6$ at low temperature, typically
taken as a hallmark of its conducting surface state, can systematically be
influenced by different surface treatments. We investigate the effect of
inflicting an increasing number of hand-made scratches and microscopically
defined focused ion beam-cut trenches on the surfaces of flux-grown
Sm$_{1-x}$Gd$_x$B$_6$ with $x =$ 0, 0.0002. Both treatments increase the
resistance of the low-temperature plateau, whereas the bulk resistance at
higher temperature largely remains unaffected. Notably, the temperature at
which the resistance deviates from the thermally activated behavior decreases
with cumulative surface damage. These features are more pronounced for the
focused ion beam treated samples, with the difference likely being related to
the absence of microscopic defects like subsurface cracks. Therefore, our
method presents a systematic way of controlling the surface conductance.
