I have the standard subscription to Netflix and unfortunately it often happens that 3 people want to watch and then accidentally upgrade to premium. Can't you somehow turn off this ad or set a pin to change the subscription?
Just use netflix according to the terms and conditions and don't share the account.
"Accidentally" on Premium Upgrade. Ah yes
If you are on a mobile device, you can simply download the film / series, turn off Wi-Fi and enjoy the film / series. Otherwise I don't think there's such a thing.