Google Translate is suggesting 'abmelden', but I'm wondering if there's a more commonly used idiom that could go at the bottom of an email or otherwise.
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Abo kündigen. (Abo is a universally understood short form of Abonnement - subscription, kündigen is to terminate.)
Abmelden will confuse some people as it is most commonly used on the Web for "logging out" (e.g., from a forum).
I find them both inferior to my proposal, which wins on grounds of brevity.
To add some more alternatives to the nice answer given already: