The blocking effect causing the suppression of spurious frequencies in an injection-locked optoelectronic oscillator (IL-OEO) is studied in this paper. The blocking effect analysis in nonlinear systems is carried out using the Volterra series and is experimentally measured in a microwave amplifier. The blocking effect in the nonlinear system helps in analyzing the suppression of spurious frequencies in an IL-OEO. Under injection locking, the spurious frequencies in the IL-OEO are significantly suppressed, and the spurs are further suppressed when the power of the reference signal is increased.