  Femtosecond time-resolved mid-infrared reflectivity is used to investigate
the electron and phonon dynamics occurring at the direct band gap of the
excitonic insulator Ta$_2$NiSe$_5$ below the critical temperature of its
structural phase transition. We find that the phonon dynamics show a strong
coupling to the excitation of free carriers at the \Gamma\ point of the
Brillouin zone. The optical response saturates at a critical excitation fluence
$F_C = 0.30~\pm~0.08$~mJ/cm$^2$ due to optical absorption saturation. This
limits the optical excitation density in Ta$_2$NiSe$_5$ so that the system
cannot be pumped sufficiently strongly to undergo the structural change to the
high-temperature phase. We thereby demonstrate that Ta$_2$NiSe$_5$ exhibits a
blocking mechanism when pumped in the near-infrared regime, preventing a
nonthermal structural phase transition.
