Getting a Lawyer for Long Term Lets

I feel that he conned us, to con, C-O-A-N is to deceive. What happened was it was the end of the contract and me and my flatmates wanted our deposit back. He had stapled a really tacky picture on like a wooden wall which we never liked but there it was. When we went to leave, he had taken the picture off him, not us. And under the picture was some graffiti, which the people before us had clearly written. It just so happened that it stapled the picture over it. Very hard to prove as well. Impossible. But he had the money and he kept it. That made me very angry