Landlord playing tricks not to give back deposit (Cologne)

Hi, I need your advice. Obviously our landlord does not want to give back the deposit. I gave him the keys to be able to show the flat to people interested in it since we moved to another city. Now he is always around, talking badly about us to the guy we hired for the renovations, hiding or taking away his material when he's not there, destroying the renovation works,... how can I stop him?
It would be good if some of you came over to Gaußstr. 8 in Cologne-Mulheim and confirmed that the flat is in a condition that allows the next tenant to move in.