  We synthesized a series of K0.8Fe2-xCoxSe2 samples with nominal compositions
0\leq x\leq 0.035 and investigated their physical properties. The results show
that the superconductivity in K0.8Fe2-xCoxSe2 is quenched down to 5 K by 0.5
at. % Co doping, the fastest quenching rate ever-reported. The role played here
by Co is in contrast with the one in FeAs based superconductors where Co
usually induces superconductivity from parent compounds. Such a rapid quenching
favors a localized 3d model against the itinerant one for iron pnictide
superconductors.
