  Within an isospin- and momentum-dependent transport model using as an input
nucleon density profiles from Hartree-Fock calculations based on a modified
Skyrme-like (MSL) model, we study how to eliminate the influence of
neutron-skin size difference of initial colliding nuclei in probing the nuclear
symmetry energy. Within the current experimental uncertainty range of
neutron-skin size of $^{208}$Pb, the Pb+Pb collisions are performed in
semicentral and peripheral collisions with impact parameters of 5 and 9fm and
at beam energies from 50 MeV/nucleon to 1000 MeV/nucleon, respectively. It is
shown that combination of neutron and proton collective flows, i.e.,
neutron-proton differential elliptic flow, neutron-proton elliptic flow
difference, neutron-proton differential transverse flow and neutron-proton
transverse flow difference, can effectively eliminate the effects of
neutron-skin size difference and thus can be as useful sensitive observables in
probing nuclear matter symmetry energy in heavy-ion collisions. Moreover, the
combined neutron-proton stopping power including the neutron-proton
differential stopping power and neutron-proton stopping power difference can
also eliminate the effects of neutron-skin size difference and shows some
sensitivities to symmetry energy especially at the lower beam energy.
