Devise: How to operate on individual accounts?

Hi everyone.
I’m trying to solve the problem described here - and I don’t quite get how devise handles sessions: why does it always refer to scopes? How do I sign out just one user for example?

And I would appreciate if you share links to rails apps code examples where devise is used and some custom actions are implemented on user accounts e.g. showing all users, deleting user, and so on.