The interaction of beam ions with absolutely unstable electrostatic ion cyclotron waves in an ion beam-plasma system is examined. Although a single wave model has been successful in predicting the initial linear growth, the nonlinear saturation, and the early stage of the amplitude oscillation of the unstable wave, it does not predict the subsequent nonlinear wave behavior. The authors numerically examined this behavior as the interaction of beam ions with unstable waves consisting of many waves. Then, at the subsequent nonlinear stage, beam ions is scattered by the interaction with many waves, and the average energy of the beam slows down. As a result, the amplitude oscillation of the unstable wave collapses, and then waves decay.