What should I do with the spray paint?

My grandson (he is 19) is living with me temporarily. With his things he brought 3 cases of spray paint. I took the spray paint away because he is 'tagging' (I feel), he is on probation for 3 other offenses and in violation of probation. I realize I cannot change him, it has to come from within but I believe he is heading down a wrong path. He wants the spray paint back - I said no. what should I do??