Spam users on drupal7 site being created automatically
How to prevent it from happening you already know:


Anti-spam tools like Mollom
Captcha


For mass deletion, use Views to list them:


Create a new users view (add new view, show: "Users"),
add relationships you feel will be helpful to identify spammers, like "User: Content authored".
add filters based on these relationships, like


Created unreasonably high amount of content or no content at all,
only users with account younger than the spam wave,
et cetera;

use VBO to actually delete unwanted ones - just check them, or use select all and uncheck good guys. Of course good filters will reduce number of good gu