Jet ignores request to close account for a 73 year old after settling

Jet Store would not close my 73 year old pensioner mother! I wrote to them and asked for a settlement letter, which they were quick to send. The amount was paid and I sent POP and request to close the account (this was done three times). We have not heard anything from Jet but sms saying the account is now in arrears.

This deliberate ignoring instructions from customers is both ******** and *********.

Please help