Send all tenants a letter telling them if the damage continues they will either lose their garage priviledges or when the leases are up for renewal you will raise them to cover new doors. It's either "them" (same term used in a 1950's B movie about big irradiated ants), their friends or their kids. So raise the rents! They will either start paying real close attention to damage going on or move. You may also find that *if* you have some good renters they will monitor the other trash and tell you who it is. Either way you're ahead.