We study the frequency dependence of the dielectric function ϵ(ω,T) of a Josephson junction array in which sites or bonds are randomly removed with a given probability 1-p. The following two regimes are considered: a) the spin wave contribution to ϵ is calculated - both for underdamped and overdamped motion - in the coherent potential approximation. The zero frequency spin wave stiffness goes to zero at a critical value of p. Depending on frequency the dynamics is governed by extended waves or by more localized fracton modes. b) In the continuum approximation the motion of one vortex is modeled by its potential energy in a fixed distribution of circular holes. The vortex mobility is then calculated by using Mori's procedure for evaluationg dynamic correlation functions.