My site has a bustling Classified section. There are a handful of scammers who continually prey on naive members. We've provided best practice, tips and tricks for our members, and of course we've blocked the scammers via IP addresses. Since the scammers use VPN they keep registering new accounts. Cookies won't work as they'll just delete them before re-registering. 2FA via an app won't work as it will deter new registration. I've inquired as to why there isn't a register members via SMS add-on (like all the "BIG" SM sites).