The exclusion of mixed electrolytes from charged and uncharged cylindrical capillaries is studied using the grand canonical Monte Carlo method and the Poisson-Boltzmann equation. Concentration profiles inside the capillary, the Donnan exclusion coefficient, the separation factor, and concentration fluctuations were evaluated for a system with a mixture of divalent and monovalent counterions and monovalent co-ions. The electrolyte is treated in the restrictive primitive model approximation. All these quantities are studied as a function of the composition of the bulk external electrolyte. The calculations apply to a range of surface charge densities and to two values of the total ionic strength. The simulation results are compared with the predictions of the Poisson-Boltzmann approximation and the ideal Donnan theory. Both approximate theories grossly overestimate the ability of a micropore to exclude electrolytes. For higher concentrations of divalent counterions present in the system the Poisson-Boltzmann theory yields incorrect results. We present a comparison with the experimental data for charged membranes immersed in a mixed KCl/CaCl2 solution.