Ab initio and kinetic Monte Carlo calculations elucidate the electronic nature of surface Sn alloying on the stability and mobility of a Cu adatom on the Cu-Sn (111) alloy surface. Sn atoms segregate on the surface and introduce forbidden areas around them within which adatom adsorption is strictly prohibited. In addition they reduce dramatically both the binding and the mobility of Cu adatoms in neighboring adsorption sites outside the forbidden areas, in contrast to experimental suggestions. Thus, Sn atoms act as blocking sites inhibiting the Cu adatom diffusion. The underlying mechanisms are the structural deformation associated with the oversized Sn atoms and the enhancement of the adatom-surface interaction in the vicinity of Sn atoms.