  We use axisymmetric magnetohydrodynamic simulations to investigate the
spinning-down of magnetars rotating in the propeller regime and moving
supersonically through the interstellar medium. The simulations indicate that
magnetars spin-down rapidly due to this interaction, faster than for the case
of a non-moving star. From many simulation runs we have derived an approximate
scaling laws for the angular momentum loss rate, \dot{L} \propto
\~\eta_m^{0.3}\mu^{0.6}\rho^{0.8}{\cal M}^{-0.4} \Omega_*^{1.5}, where \rho is
the density of the interstellar medium, \cal M is Mach number, \mu is the
star's magnetic moment, \Omega_* is its angular velocity, and \eta_m is
magnetic diffusivity. A magnetar with a surface magnetic field of 10^{13} -
10^{15} G is found to spin-down to a period P > 10^5-10^6 s in \sim 10^4 - 10^5
years. There is however uncertainty about the value of the magnetic diffusivity
so that the time-scale may be longer. We discuss this model in respect of Soft
Gamma Repeaters (SGRs) and the isolated neutron star candidate RXJ1856.5-3754.
