We investigate the doping behavior in the ultrathin part of individual free standing spicular or conic silicon nanowires (NWs) by measuring the local resistivity of the NWs using scanning spreading resistance microscopy. The NWs are boron-doped and the doping efficiency is found to dramatically decrease as the NW diameter is below 25 nm. Our experimental results reveal the dependence of the acceptor deactivation on the diameter of the NW cross section. The deactivation mechanism is discussed by comparing the experimental data with theoretical models considering the dopant deactivation induced by carrier traps at the Si/SiO2 interface and due to the dielectric mismatch.