  The motion of a mechanical resonator is intrinsically decomposed over a
collection of normal modes of vibration. When the resonator is used as a
sensor, its multimode nature often deteriorates or limits its performance and
sensitivity. This challenge is frequently encountered in state-of-the-art
optomechanical sensing platforms. We present a mechanical design strategy that
ensures that optomechanical measurements can retrieve information on a single
mechanical degree of freedom, and implement it in a sliced photonic crystal
nanobeam resonator. A spectral design approach is used to make mechanical
symmetries robust against practical disorder. The effectiveness of the method
is evaluated by deriving a relevant figure of merit for continuous and pulsed
measurement application scenarios. The method can be employed in any mechanical
design that presents unwanted spurious mechanical modes. In the nanobeam
platform, we experimentally show an increase of the signal to noise ratio of
the mode of interest over the first spurious mode by four orders of magnitudes.
