The police report is for if they manage to get anything with your cards before they were cancelled. Any time I've had or known someone else who had a card number used the bank requires the police report number to cancel the charges.
It took under 10 minutes to cancel all the cards, and we asked each card supplier if there were any transactions, because if they were, they were fraudulent. There were no transactions. Also, you do not need a police report cancel charges, I used to work with a credit card issuing bank (a client) and learned all about the fraud and liability laws. Protip: Never use a debit card for anything but as your ATM card (if that's how your bank doe it).