Unauthorised Debit from Liquid Capital

I had a service plan with them of which I settled in Dec2017 and they confirmed in B&W that it was fully paid up. January2018 went well no problems, Feb2018 they debited my account without my authorization, I emailed them and they said it was an error from their side and they processed the refund after 14days. Mar2018 they went again and debited my account, why? I do not know, who gave them an authority to debit? I do not know - how as a person make sure my money its safe in the bank account if people can just go and dib in for as much as they want?