  The insertion of a magnetic $\pi$ flux into a quantum spin Hall insulator
creates four localized, spin-charge separated states: the charge and spin
fluxons with either charge $Q=\pm1$ or spin $S_z=\pm1/2$, respectively. In the
presence of repulsive Coulomb interactions, the charged states are gapped out
and a local moment is formed. We consider the Kane-Mele-Hubbard model on a
ribbon with zigzag edges to construct an impurity model where the spin fluxon
is screened by the helical edge liquid. In the noninteracting model, the
hybridization between fluxon and edge states is dominated by the extent of the
latter. It becomes larger with increasing spin-orbit coupling $\lambda$ but
only has nonzero values for even distances between the $\pi$ flux and the edge.
For the interacting system, we use the continuous-time quantum Monte Carlo
method, which we have extended by global susceptibility measurements to
reproduce the characteristic Curie law of the spin fluxon. However, due to the
finite extent of the fluxons, the local moment is formed at rather low
energies. The screening of the spin fluxon leads to deviations from the Curie
law that follow the universal behavior obtained from a data collapse.
Additionally, the Kondo resonance arises in the local spectral function between
the two low-lying Hubbard peaks.
