I don't think it would be easy to write a global one for every site, but I think it would be cool to make a community-driven unsubscribe script collection.
While there would be some positive aspects of something like that, you'd either have to have it moderated strictly by a trusted moderator or make sure every user understood every line of all code they were executing from the site.
The potential for scripts that did very bad things is immense, especially given how easily you can massively obfuscate JavaScript code.