As per your information you mentioned "as per mutually agreed terms". Are these terms written? If yes, then you may have some leverage regarding payment terms. If there's no agreement letter, this may be a lesson for you that in future any transactions that involves monetary must always have agreement paper signed by both parties.
Just my suggestion, since they belong to the worst client, and if in your agreement letter you have mentioned you have the right to cancel the terms anytime it deems necessary, then you may cancel this agreement and inform them that in future there will be no support to be provided.
For the pending payment, use your agreement letter terms and conditions to sue them. I'm just not sure how the system works in your country. Though this may take some financials, but there are penalties I guess in which they call "indemnification". It is a part of an agreement that provides for one party to bear the monetary costs, either directly or by reimbursement, for losses incurred by a second party (What is indemnification? - Definition from WhatIs.com). In future you might consider to include this in your agreement clause with the % cost of losses. But best ask a lawyer prior to finalizing any agreements.
From Bangladesh, Dhaka
As Shallohal has brought to fore "mutually agreed terms" is vague term. These mutually agreed terms must be converted into contract agreement. Under the provisions of this agreement, purchaser has to issue Purchase Order (PO) to provide the material or the services, as applicable. On rendition of the services, service provider is expected to obtain raise his invoice. On completion of the credit period or even before completion of the credit period, service provider may approach for the clearance of his invoice.
Did you follow the above-mentioned universal procedure? If not, then please confirm what was missing out of the above procedure.
From India, Bangalore