Debit Order loaded without authorisation

we have been with Bonitas for more that 10 years, for the last 5 years we have been paying our membership via eft (cash), about a month ago bonitas contacted us wanting banking details for a refund, we provided them with one of or saving accounts number, which we don't actually use. Bonitas loaded these banking details for debit order for our contributions??? we did not authorize them to do that. 1st of October they debit the account and we get rd by the bank, because there is no money in that account. immediately they suspend the medical without letting us know about it... now the rd reflects bad on my account and who is going to pay for the fees????