Is it illegal for a tenant after the place I moved out of to use my utility account for 1+ year w/o permission?

I forgot to shut off the account when I moved, didn't find out that the account had been used due to moving out of state and not receiving bills. I have contacted the landlord, naturally they won't tell me the tenant - who still lives there- name. I've been trying to get ahold of the tenant for 2+ months. It's stated in your lease what utilities are to be set up in your name INCLUDING the account of mine they used. Small claims court won't let me file since I don't know their name. Is this considered identity theft? What can I do? We received a 10 day shut off notice for this account because we are unable to pay the $600+ PAST racked up bill they accrued.