You can instruct MediaWiki to require verification of
email addresses
entered in user preferences. With a little hacking, you could modify
the login functions in User.php to check the user's validation hash,
and see if they've been validated. If not, then you would reject
login.
One more question in regards to this.
How can you enforce people to fill in their email address?
I haven't been able to find anything on that yet.
Mischa