Unfortunately they are right. You should have cancelled the old policy if you no longer wanted it.
In the event of a claim they actually would have paid out. Unlike house insurance where the claim would not have been paid twice, you can have multiple life assurance policies. If there was a claim, the insurance company would have been the worst in the world had they cancelled the policy without you telling them.
Even if you had cancelled the direct debit, because life assurance policies are sometimes assigned to lenders, the bank sometimes will not cancel the direct debit if there is another interested party in the policy, in my opinion however they should have told your wife this and advise her to ask the lender to release their assignment.
Without proof that you cancelled the direct debit, there is little you can do to get the money back from the bank. As the insurance company stated, if there had of being a claim they would have paid so you did pay for a service whether you realised it or not.
If it was only a month or two's payment, the life company may have written it off and given you a refund but the fact it went for years, they would argue why you didnt realise it when you got your bank statements.
There isnt a lot you can do except write it down to experience and vow to check your bank statements going forward. Sorry I cant give you advice to the contrary but I dont see you have any choice here but to accept that the money is gone.
www.CheaperLifeAssurance.ie