semi-analytic model of galaxy formation (Menci et al. 2004a; 2005), which includes the growth of SMBHs through gas accretion triggered by galaxy encounters. The AGN feedback we include in the present paper is substantially different from the implementations in Croton et al. (2006), Bower et al. (2005) and Cattaneo et al. (2006), since it is produced during the short AGN active phase, sweeping the cold gas content of the galaxies in a way similar to that resulting in the simulations by Di Matteo et al (2005); in addition, since it is associated with the observable active phase of AGNs, the feedback effect we investigate is mainly produced at high redshift. Before exploring the impact of such an impulsive form of AGN feedback on galaxy formation, we first test against observations the predicted evolution of the luminosity functions of QSOs from $z=4$ to the present, since the feedback we consider is directly related to their emission. After checking the consistency with the observed local $m_{BH}- \sigma$ relation between the BH mass $m_{BH}$ and the one-dimensional velocity dispersion $\sigma$ of the host galactic bulges, we compute the galaxy colour distribution at low ($z\lesssim 0.1$) and high (up to $z=3$) redshifts, discussing the 