If you are sure that all of the blocked accounts need removing, then you could run a query in phpmyadmin (or your mysql admin of choice):
DELETE FROM users WHERE `status` = 0 AND `uid` != 0;
That should get rid of all blocked accounts and leave the anonymous user. Of course you would DEFINITELY want to back your DB up first. Also, realize this also get rid of any accounts that may be legitimate, but that you may have set status as 'blocked' for some reason.
Note: I should say that I know way more about 6 than 7. So if the users table is drastically different in Drupal 7, this may be incorrect.