Non adherence to set off

I am having a personal loan with capfin which I have topped as the need arises. Last year Nov I couldn't pay my Nov instalment. One of their T&C's is the set off which I am aware of, however they keep on forcing a debit order every month since December and this set off are Dishonored because there's no enough funds, by the way I'm still paying their normal installment. As per the NCA before they do a st off they must be sure that there's funds . I have already had 3 set off which returned and they are impacting my cheque acc negatively and my account is about to be closed.