Older adults are increasingly targeted by online fraudsters, losing millions of dollars each year to scams in the United States alone. This paper systematically reviews efforts to prevent these scams across multiple domains of research. We apply and expand upon an existing categorization of online fraud prevention efforts to describe the available research. Additionally, we identify and group both the vulnerabilities of older adults targeted by fraudsters and the attacks defended against. Finally, we identify areas for future research in this growing field.