How do I remove the scam pop-up telling me to call MacOS support?

Hi! My elderly Mum has the incredibly annoying pop-up on her Mac, telling her to contact MacOS support. We know this is a phishing attempt, so we haven't responded in any way. There's also a terrible (that is, non-stop) repeating verbal message.

Mum has powered off her computer. Will the pop-up still be there when she starts up her computer later on?

What do we do to prevent this happening again?

Many thanks!