This question has been answered by BobRay. See the first response.
You could delete the user and have them re-register. I don't know of any other way to do it. It's the Register snippet that sends the activation email, and it will only do so after a user fills out the registration form.
I guess you might be able to write your own custom form that let you input the user ID, and on submission, got the user information, deleted the user, and sent them a new email containing a new activation link, but it would be a lot of work, a royal pain to test, and not worth it, imo, since not many users will need it.