Email verification for new users

I would like to implement a plugin that requires new users to reply to an email, to verify their email address, on registration. I am already doing extensive codex research, but I am very new and would appreciate some hints, especially regarding keeping the user inactive until they have verified their email. The rest I think I will manage on my own.

User Without Email?

When creating a user in WordPress an email address is required for that user. The website I’m working on requires that users remain anonymous (it’s part of a research study) and I’d consider email to be an identifying piece of data. Users will be added manually, so email is not required to confirm accounts or prevent spam sign-ups.