  The notion of a finite pairing interaction energy range suggested by Nam,
results in some states at the Fermi level not participating in pairings when
there are scattering centers such as impurities. The fact that not all states
at the Fermi level participate in pairing is shown to suppress $T_c$ in an
isotropic superconductor and destroy superconductivity. We have presented
quantitative calculations of $T_c$ reduced via spinless impurities, in good
agreements with data of Zn-doped YBCO and LSCO, respectively. It is not
necessary to have the anisotropic order parameter, to account for the
destruction of superconductivity via non-magnetic impurities.
