I think I remember that VB or phpBB does something like that with posts. Preventing a person from posting more than one post in a given timeframe which had an adjustable setting in the admin. I am sure that it checks the active valid account of the user. Usually when mass signups occur, its a new email and user account each time. I am not sure how you could put a "limiter" in there. I have a friend who did all the code for a very large community site that got hit by DOS around its launch time. They dont use a character recognition method on the signup page. I am going to see if he made changes to how accounts are activated on his site. I also know his site uses all of his own custom PHP as well as some C+ to help speed page load times. Its a very graphically site and when it lauched suffered from slow page load times, even with a broadband connection.
If anyone cares to check it out, go here