In this paper an active vibration controller for flexible systems is developed using a sliding manifold approach. The controller is designed to "eliminate" the system modes whose frequencies are close to the disturbance frequencies. The sliding surface consists of a time-invarying part which "eliminates" the selected modes without affecting the remaining dynamics, and a time-varying part which bounds the control amplitude and its rate in the transient. Moreover, the features of the controller allows one to reduce the excitation of the high-frequency modes to reduce the spillover phenomenon.