  High-quality K(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals have been grown by
using KAs flux method. Instead of increasing the superconducting transition
temperature $T_{\rm c}$ through electron doping, we find that Co impurities
rapidly suppress $T_{\rm c}$ down to zero at only $x \approx$ 0.04. Such an
effective suppression of $T_{\rm c}$ by impurities is quite different from that
observed in Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ with multiple nodeless
superconducting gaps. Thermal conductivity measurements in zero field show that
the residual linear term $\kappa_0/T$ only change slightly with $3.4\%$ Co
doping, despite the sharp increase of scattering rate. The implications of
these anomalous impurity effects are discussed.
