Call their bank to cancel, then cut it up and throw it away.
I found a wallet and called Chase bank (their card) and they cancelled the card, but wouldn't contact the guy to tell him I had his wallet. I ended up going to his house about 30 miles away. He had his license, Social Security card, and about $50 in cash in it. He gave me the cash, which I profusely tried to refuse but he insisted.