  We discuss transport through an interferometer formed by helical edge states
of the quantum spin Hall insulator. Focusing on effects induced by a strong
magnetic impurity placed in one of the arms of interferometer, we consider the
experimentally relevant case of relatively high temperature as compared to the
level spacing. We obtain the conductance and the spin polarization in the
closed form for arbitrary tunneling amplitude of the contacts and arbitrary
strength of the magnetic impurity. We demonstrate the existence of quantum
effects which do not show up in previously studied case of weak magnetic
disorder. We find optimal conditions for spin filtering and demonstrate that
the spin polarization of outgoing electrons can reach 100%.
