You dont even need a mod, a mod just makes it easier.
All the information is stored in the database in plain text, just browse through the tables, connect the user ids to the message IDs, and thats it.
Admin ToolKit v2.1a - An Admins most helpful tool for user management. Now Supports Mass User Deletion!
Change User's: names, passwords, emails, active status and avatar/pm permissions.
Ban/Unban Users, change Post and Resync Counts, and promote/demote users to admin.
Completely independent from your phpbb user account settings. No installation required, just upload one file. User Upload ToolKit Beta - A quick and easy, 30 second-install, attachment mod. Now Supports Dynamic Thumbnails!