Being harassed for payment already made

I’ve spent been MONTHS trying to get my account updated with my payment which was made through their online payment gateway. Payment was never allocated to my account and I have been bombarded with threatening texts, calls and emails despite my endless efforts to get my payment allocated and the problematic error on their part. I’ve spent hours on the phone and behind my laptop to try and sort this out to no avail. They just keep on requesting that I repay and move on, which out of principle I will not do. Absolutely PATHETIC and TERRIBLE.