This paper is concerned with the active control of a string. A nonlinear Kirchhoff-Carrier model of the transverse vibration of a string is considered (presenting a pitch glide at high excitation levels), controlled by a force in its domain. A finite-time tracking controller is designed for a point force, that controls one mode of the string to reference dynamics corresponding to a string with other physical parameters. We illustrate in a simulation how the controller can be used to modify the damping, resonance frequency or the presence of a pitch glide for the selected mode of vibration of the string.