HELP Isearli company trying to get Refund after work completion

Hello, I had partnered with an online Ad agency to deliver Website visitors to them. Ad Link : I have Delivered all the traffic to them on the very same day, After delivering all the traffic, i got a reply in skype from their staff "Hi we are very disappointed with your traffic. the quality is not what you told Ronen, and we want to stop the test NOW and get back the money you didn't spend yet ($1700). Of course the claim is based on the agreement you signed before the prepayment transfer." Later I didnt respond back to them, because i completely knew they were trying to scam me, Later i checked the advertising dashboard which they have, even in that they have some how manipulated the advertising link into make it count less number of visitors. and making it look like only $1300 worth of traffic was delivered. After seeing this, I didnt respond back to them, I completely blocked their 2 skype contacts. Even though they were asking for a refund. Today they have emailed me with a PDF file containing the following texts. The agreement which i used to sign them has my Full name, But not my valid signature, i used electronic signature to generate a fake signature and send them at that time because i was in a hurry. They have paid me through an online payment processor called Payoneer, From Payoneer the payment has been directly transferred to my Bank account. They have my 2 email which i used to signup to their platform. And also they dont have my real Address, I had added a fake address. They are threatening me that they are going to file a legal complaint in the court and would request more money. I am really helpless in this stage because i dont have the proof now, because i had delivered the traffic 6 months back. and now after 6 month, they emailed me with this. I am worried, I don't have money to go to the court, because i have worked hard to deliver the service to them, now after some manipulation from their end, suddenly after delivering the traffic they reply in skype and later after 6 months they send a threat to refund. Please give me your opinion on this, How can i protect myself? They only have my 2 Email ID, and my Full name But they can get my address and other things from my Payoneer account. I guess. And also the company is from Israel Please suggest. Thank you! This is the email which i received from them, It is in a PDF file, with signature of the lawyer, " We have availed to you a prepayment in the sum $3000.00 USD of which was to be applied to our advertising payments, however we have terminated this activity and have a balance of $1704.33 USD which was not used (the "Unused sum"). You are holding the unused sum without Despite our previous reminders, We still have not received any payment or answer from you with respect to the repayment of the Unused sum, which is held by you in violation of our agreements and without any legal merit. Therefore, we regret to inform you that if we do not obtain the payment of Unused sum in full before 09/11/2016 (deadline), we will have no other alternatives but to undertake court actions against you in order to retrieve the Unused sum along with all costs and expenses related thereto. Should your payment not be received in full within the next seven days starting from 03/11/2016 to 09/11/2016, immediate court actions will take place. We would advise to treat this matter with urgency and we believe you will not ignore this final reminder. To save yourself the inconveience of court actions, you are required to settle the total ammount of debt and additional costs as soon as possible. We still trust court actions will not be necessary and look forward to receiving payment by return This letter is without prejuice to any rights and remedies which are all fully reserved. " Please Help Thanks