Can't stop debit order and can't arrange a payment plan via eft

I've called Capfin numerous times and advised them I'm not working anymore and this no money will be in my bank account for them to debit, however, I've tried to make arrangements to pay an installment via eft. Capfin will accept this however they won't stop my debit order. My bank account is running badly in the negative due to insufficient funds. I cant pay Capfin via eft an maintain a good record on my bank account. This is unfair of Capfin. I advised them to please stop the debit order but they keep on saying they can't. I've told them I can't pay an istallment to them and cover my bankfees. They don't seem to care. Now I can't pay them as I'm trying to pay money in my bank account account with all the negative figures due to insufficient funds. Now Capfin won't be able to get an installment from me at all. Customer service is extremely bad. Get an email from different people every time and have to explain the same question to everyone everytime.