Hathway — False allegations for Modem submission

I was a user from Hathway for 1 year. When I was out of town I was getting ton of calls that please recharge your account. I informed them multiple times that I am out of town and won't be able to recharge. After some days they started harrassing by saying "Agar paise ni h to lete ku ho WIFI" . After which I asked the lady to take your modem and I want to end the services with Hathway.
A hathway engineer came and took modem and cutted the wire as per there procedure and said that my account will be disconnected and after which I won't get any calls.
I didn't got any calls from 6 months and I thought my account is disabled now so I was okay but today i got the pathetic email saying I have to submit the modem otherwise they will sue me.
I have already submitted the modem to hathway engineer so how can I submit new router?
Account No - [protected]

Never seen such a pathetic company and their false allegations. I have seen on google that they are doing it with their ex customers in order to harras and recharge again.