keeping funds that do not belong to their client

My problem with standard bank is that they recycle account numbers. i closed my account in 2006, and discovery made a refund claim into my old closed standard bank account. this happened in march 2015 and at this time apparently standard bank was updating their system. so i assumed the money was not rejected by them due to the system update, after a month of calling the call centre going the westville branch standing in endless queues at enquiries, i was told the account has been allocated to someone else and they can do nothing about getting back the money, it is up to discovery to give me my money. they are not even willing to talk to their client who has my money. all i have from them is a letter stating that the acc number in question does not belong to me and that it is an active account. are no measures taken to ensure the correct person is paid. is this not theft, and does standard bank sanction this thieving of their clients? as it stands i am R700 poorer and neither institute cares that what has transpired is grossly unfair and unjust.