Unprofessional Service.

I had a 3 months plan to settle an account that was handed over to them for collection. Feb 2020 debit order successful, March 2020 according to them insufficient funds. No one advised thereof which was not true. April 2020 debit order successful. All went well. June I received a letter of demand with an enormous outstanding amount which I have disputed. July they deduct money from my account without my permission which I had reversed as I did not accept a debi-check from them. I have sent numerous emails to them with little or no resolution to this matter. Been harassed every single day by about 5 consultants at a time. I just want clarification from top management and for the harassment to stop.