  In an ultracold, optically trapped mixture of $^{87}$Rb and metastable
triplet $^4$He atoms we have studied trap loss for different spin-state
combinations, for which interspecies Penning ionization is the main two-body
loss process. We observe long trapping lifetimes for the purely quartet
spin-state combination, indicating strong suppression of Penning ionization
loss by at least two orders of magnitude. For the other spin-mixtures we
observe short lifetimes that depend linearly on the doublet character of the
entrance channel. We compare the extracted loss rate coefficient with recent
predictions of multichannel quantum-defect theory for reactive collisions
involving a strong exothermic loss channel and find near-universal loss for
doublet scattering. Our work demonstrates control of reactive collisions by
internal atomic state preparation.
