  The gravitational wave event GW170817 with a macronova/kilonova shows that a
merger of two neutron stars ejects matter with radioactivity including
$r$-process nucleosynthesis. A part of the ejecta inevitably falls back to the
central object, possibly powering long-lasting activities of a short gamma-ray
burst (sGRB), such as extended and plateau emissions. We investigate the
fallback accretion with the $r$-process heating by performing one-dimensional
hydrodynamic simulations and developing a semi-analytical model. We show that
the usual fallback rate $dM/dt \propto t^{-5/3}$ is halted by the heating
because pressure gradients accelerate ejecta beyond an escape velocity. The
suppression is steeper than Chevalier's power-law model through Bondi accretion
within a turn-around radius. The characteristic halting timescale is $\sim
10^5$--$10^9$ sec for the GW170817-like $r$-process heating, which is long
enough to continue the long-lasting emission of sGRBs. The halting timescale is
sensitive to the uncertainty of the $r$-process. Future observation of fallback
halting could constrain the $r$-process heating on the year scale.
