  A concept of \emph{isospin blocking} in the process of isospin diffusion in
heavy-ion collisions is raised. Generally, in the process of isospin diffusion,
isospin asymmetry would diffuse from the place with large asymmetry to the
place with small asymmetry. However, our study shows that the isospin diffusion
could be blocked in case the local value of the symmetry energy is larger. We
dub this phenomenon ``isospin blocking''. To check this behavior, in the
framework of an Isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport
model, isospin diffusions in the isotope Sn+Sn reactions at 270 MeV/nucleon are
studied. It is shown that the value of the after-diffusion asymmetry is
distinctly blocked if the local symmetry energy is large. The effects of the
isospin blocking on the isospin asymmetry of dilute and dense matter and the
final $\pi^-/\pi^+$ ratio in heavy ion collisions are demonstrated.
