They might have changed. When I checked just now, they offer a phone number and an E-Mail address to cancel a physical paper subscription (there's no account, so that makes sense).
An online "Spiegel+" subscription can be cancelled via their website.
I live in Germany and have cancelled several physical magazine and newspaper subscriptions and even political party memberships via e-mail after signing up online. I can't say anything about Der Spiegel but I would be surprised if they did it any different given that German consumer protection agencies have some teeth.
They might have changed. When I checked just now, they offer a phone number and an E-Mail address to cancel a physical paper subscription (there's no account, so that makes sense).
An online "Spiegel+" subscription can be cancelled via their website.
https://abo.spiegel.de/de/c/abo-service/spiegel-abo-kuendige...
It may be different for their non-German publication, but I had trouble finding any English information - which may be saying something...