  We demonstrate the control of $^4$He(2$^3$S$_1$)-$^7$Li(2$^2$S$_{1/2}$)
chemi-ionization reactions by all-optical electron-spin-state preparation of
both atomic species prior to the collision process. Our results demonstrate
that chemi-ionization is strongly suppressed (enhanced) for non-spin-conserving
(spin-conserving) collisions at thermal energies. These findings are in good
agreement with a model based on spin angular momentum coupling of the prepared
atomic states to the quasi-molecular states. Small deviations from the model
indicate the contribution of the $^4\Sigma^+$ channel to the reaction rate
which is in violation of spin conservation.
