WordPress: Fake user registrations

For the past couple days I've noticed a rash of fake user account's being created.  I wonder if this isn't meant to be some new way to possibly exploit WordPress.

I've kept the original emails I received from WordPress, and am very tempted to delete the accounts.  However, WordPress only seems to allow the deletion of accounts.  What if I want to 'ban' an account/username/email?

I suppose I'll have to go find a plugin, or build one myself.

I've Akismet installed, so there's been no spam yet (other than the usual questionable ones that get reported), but ...

Update: December 16, 2006

The common thread was all emails contained the web.de domain.

Update: July 4, 2007

The solution was to ultimately stop registrations.