  We present a theoretical analysis of two-dimensional Dirac-Rashba systems in
the presence of disorder and external perturbations. We unveil a set of exact
symmetry relations (Ward identities) that impose strong constraints on the spin
dynamics of Dirac fermions subject to proximity-induced interactions. This
allows us to demonstrate that an arbitrary dilute concentration of scalar
impurities results in the total suppression of nonequilibrium spin Hall
currents when only Rashba spin-orbit coupling is present. Remarkably, a finite
spin Hall conductivity is restored when the minimal Dirac-Rashba model is
supplemented with a spin-valley interaction. The Ward identities provide a
systematic way to predict the emergence of the spin Hall effect in a wider
class of Dirac-Rashba systems of experimental relevance and represent an
important benchmark for testing the validity of numerical methodologies.
