  We study spectra generated by Comptonization of soft photons by cold
electrons radially free-falling onto a black hole. We use a Monte Carlo method
involving a fully relativistic description of Comptonization in the Kerr
space-time. In agreement with previous studies, we find that Comptonization on
the bulk motion of free fall gives rise to power-law spectra with the photon
index of Gamma >~ 3. In contrast to some previous studies, we find that these
power-law spectra extend only to energies << 511 keV. We indicate several
effects resulting in generic cutoffs of such spectra at several tens of keV,
regardless of any specific values of physical parameters in the model. This
inefficiency of producing photons with energies > 100 keV rules out bulk motion
Comptonization as a main radiative process in soft spectral states of
black-hole binaries. The normalization of the power law (below the cutoff) with
respect to the peak of the blackbody emission of the surrounding disc is
typically very low, except for models with an overlap between the disc and the
plasma, in which case the spectra are very soft, Gamma >~ 4.
