[dkulchenko]

A kind of hackish, but simple way:

Log into your MySQL database with php_myadmin, go to the <db-prefix>_users table, go to Browse view, and simply select each user you want to delete with a checkbox, then press the X at the bottom of the screen. phpmyadmin lists the username, real name, id, and so on of the user, so that you can quickly delete by names.

NOTE: Of course, only do this if you know what you’re doing. Using phpmyadmin, you can do anything from change one letter of someone’s username to trash your entire installation. Do not use this tool if you’re a noob.

Also, this only works if the user accounts do not have any posts associated with them. If they do, this may cause very unpredictable behavior. Before doing this, delete all the posts associated with the user accounts from bbPress, or by deleting posts from the bb_posts table. Then, you can safely delete the users using the steps above.