The n-type MNiSn (M = Ti, Zr, or Hf) half-Heusler compounds are reported as promising medium- and high-temperature thermoelectric materials; however, their p-type counterparts have suffered from poor performance due to the in-gap state caused by Ni occupying the tetrahedral interstitials. Inspired by recent findings that thermoelectric performance can be enhanced without substantially increasing compositional or structural complexity, the study attempts to manipulate the Ni interstitial defects by altering the stoichiometric composition. The results show that when HfNiSn is prepared by a non-equilibrium method, the intrinsic Ni defects are effectively suppressed by simply reducing the nominal Ni content. The suppression of Ni defects not only leads to a larger bandgap, but also attenuates carrier scattering to achieve higher mobility. After further optimization of the carrier concentration, the p-type HfNi_0.85Co_0.05Sn achieves a maximum power factor of 3100 µW m⁻¹ K⁻² at 773 K and a peak zT of ≈0.7 at 973 K, both of which are superior to that of the state-of-the-art p-type MNiSn. The results demonstrate that the off-stoichiometric ratio is effective in decoupling electron-phonon transports of thermoelectric materials with massive intrinsic defects, and also contribute to understanding the role of defect modulation in optimizing thermoelectric properties.