Using a schematic hydrodynamic description of relativistic heavy-ion collisions in the regime of complete nuclear stopping, we study the freeze-out and strange particle energy spectra from a hadronized quark-gluon plasma. We argue that characteristic changes in the slopes of the ${\mathrm{K}}^{+}$ and ${\mathrm{K}}^{\mathrm{\ensuremath{-}}}$ meson energy spectra are a signature for the quark-gluon plasma phase transition.