  We use two-color pump-probe spectroscopy to study Rydberg excitons in Cu$_2$O
in the presence of free carriers injected by above-band-gap excitation. Already
at plasma densities $\rho_\text{eh}$ below one hundredth electron-hole pair per
\textmu m$^{3}$, the Rydberg exciton absorption lines are bleached while their
energies remain constant, until they finally disappear, starting from the
highest observed principal quantum number $n_\text{max}$. As confirmed by
calculations, the band gap is reduced by many-particle effects caused by free
carriers scaling as $\rho_\text{eh}^{1/2}$. An exciton line looses oscillator
strength when the band edge approaches the exciton energy vanishing completely
at the crossing point. We quantitatively describe this plasma blockade by
introducing an effective Bohr radius that determines the energy distance to the
shifted band edge. In combination with the negligible associated decoherence
this opens the possibility to control the Rydberg exciton absorption through
the plasma-induced band gap modulation.
