If tenants ordered and paid their own utilities, then it is an agreement between them and the utility company. If you provided utilities included with the rent, or ordered it in your name, it is yours.
That is not to say that the utility company can not try to deny service to that address, which I don't believe they should be able to do. However, in that case, one route may be to pay it and take the tenants to small claims court. The other is to argue with the utility company that you weren't the one who ordered the service so they need to take it up with the tenants, which again, could put you back in small claims court or with an attorney.
All of the above is strictly my opinion and I don't profess to be an attorney.