An Andreev billiard was realised in an array of niobium filled antidots in a high-mobility InAs/AlGaSb heterostructure. Below the critical temperature T-C of the Nb dots we observe a strong reduction of the resistance around B=0 and a suppression of the commensurability peaks which are usually found in antidot lattices. Both effects can be explained in a classical Kubo approach by considering the trajectories of charge carriers in the semiconductor, when Andreev reflection at the semiconductor-superconductor interface is included. For perfect Andreev reflection, we expect a complete suppression of the commensurability features, even though motion at finite B is chaotic.