How to remove an Apple ID from a MacBook

Where exactly is it asking for your Apple Account? What is your grandfather doing when it requests it exactly?

There's 4 places an Apple Account can be logged in on a MacBook.

System Settings / Preferences ➜ Account (this is for iCloud)

System Settings / Preferences ➜ Account ➜ Media & Purchases (this is for Apple Music, Apple TV, Books and App Store Apps)

Messages App ➜ Settings ➜ @ iMessage ➜ Apple Account

and FaceTime App ➜ Settings ➜ General ➜ Apple Account

Also, any Apps downloaded with your Account from the App Store will continue to require it to update regardless of what is signed in at Media & Purchases. You would need to delete any apps you downloaded and redownload them with his account.

Without knowing what's triggering the log in prompt it's hard to say.

Did you maybe add your account to the Mail App?

Did you add for Contacts or Calendar?

What did you do back when you used his computer exactly?

Where did you look at this something for school?

That random Apple ID prompt years later is almost certainly coming from a stuck system-level credential, not from visible iCloud or Media settings. Watching an iTunes movie back then likely cached your Apple ID token inside the TV app / legacy iTunes frameworks, and on 2017–2020 Macs those tokens can survive user sign-outs, OS updates, and even device removal from appleid.apple.com. When macOS periodically refreshes DRM or background services, it tries to revalidate that old token and throws the login popup with your email. That’s why it’s random, persistent, and ignores every setting you already checked.

At this point, there are only two real fixes that actually work in the field, either create a brand-new macOS user account for your grandfather and migrate his files, then delete the old user entirely, or do a full erase and reinstall of macOS. Anything short of that won’t reliably clear the credential store. This isn’t you missing something or doing it wrong, it’s a known macOS cleanup blind spot. If he refuses both options, the popups will keep coming indefinitely.

I tried everything, deleting everything related to me from his MacBook and deleting his MacBook in my devices but after a few weeks he gets the request to log in my account and can’t do anything on his laptop before logging me back in.

Did that include the necessary steps shown in this Apple article?:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

It sounds like he missed a step.