getting rid of a large number of spam accounts

Does akismet delete spam accounts or just comment spam? A friend has a wordpress site that has more than 53.000 spam accounts on it and wants to know the fastest way to delete them and prevent more accounts from being created. I told him to install wangguard to prevent future issues but he still needs to get rid of those 53,000 accounts. I also told him to install wordfence.

Any other suggestions?