Over debit

I purchased a maths programme for 6000 and was paying via debit order. My installments was supposed to have been 11 however I have now paid 15 installments and I am almost certain they will debit me again on the 01.06.19 again. I want the extra money that I have paid and i want them to stop the debit order. I have sent them 2 emails and have not yet had any response from them. I tried calling and I get no answer. Surely they should have a system in place to stop debit orders once they are finished. Very pathetic service. Will not even consider doing business with a business like this again. I need an urgent response from this company asap.