A financial Power of Attorney is the legal document that gives you the ability to act as that person legally in most ways. It requires only the person to sign the PoA document. Using it often requires some bureaucratic hoop jumping, but that's nonetheless what it does. It doesn't prevent the person themselves from taking legal or financial actions – like giving money to scammers. For that you would need Conservatorship which is a higher level of responsibility and must be reviewed by a judge. At least, that's the situation in the US. Things may work differently elsewhere. And also, for reasons to do with the federal system, Social Security benefits are sort of their own thing and have a different process.
After my mom in law got scammed out of a few hundred euros I confiscated her laptop and moved her to a Google ChromeBook.
That's quite locked down and a few years in she still hasn't managed to ruin the OS.