ISP did not disconnect the connection after request charging rental

Please write to the service provider giving reference of your request for disconnection, asking them to stop calling you. Do not pay the rental and preserve those mails of your request for disconnection and their promise to do so within 15 days. It will be better to keep prints of these mails. They can do nothing against you. In case you want to go for litigation against harassment by the company then keep a record of all their calls, you have to also consider the costs and time involved in litigation. In case you decide to exercise option of litigation you may kindly consult in person.

If you have written communication with them asking them to disconnect then you do not have to worry. If they have asked you to pay you should write back to them that you had asked them to disconnect and they had assured you that they would do so in 15 days. Ideally you should have informed them of the continued connection after 15 days and written a strict letter to them then itself that you would not be liable to pay henceforth. If they send you any legal notice then you may contact a lawyer else you can sit back and relax.