1. Go to the bank and present a death certificate. This, coupled with some valid identification should be enough to freeze the account.
2. The next step would be to provide a probate or will. This will instruct the bank on how to proceed with the deceased person's financial affairs and any money being held in the account.
In the UK, there is also something known as a 'grant of representation' that you can apply for through the courts. This allows you to legally take over someone's finance, property and possessions. There are equivalent orders in most other countries.