Can't close my bank a/c!?

Hi, about 5 years ago I opened a bank a/c with Abbey (now Santander), about a year later I went to the store and closed it...or so I thought. Anyway, about a year ago I decided to check my credit report and this account is on there and has a staus of late repayments. As they actually failed to close my account, a direct debit kept coming out, I had moved house by this point so did not receive any post related to it. I know now I should have cancelled the direct debit with the company as well but did not know this at the time, I thought the direct debit would stop as the account is closed. When I went to see Santander about it, they agreed to bring the account to zero (as I was in debt of about �100 due to debit interest) and close the account. This year, I discovered it still was not closed, somehow it had gone into debit interest again. Again, I went to see them, they cleared the account and 'closed' it. I got a call from Santander collections department, it turns out it had STILL not been closed. I went to see them AGAIN and they issued me with a closing statement. Now, about a month later, I have received a cash card for a supposedly closed account. I am so sick of the situation as it is the only thing affecting my credit rating and I would like to buy a computer and pay for it in monthly installments. I just want to the thing gone!!! But I don't believe for a minute that if I go and see them, yet again, that it will resolve the situation. What do I do???