This is what the
manual says:
Looks like there are a couple of ways you can accomplish it:
3.5.3: User Management
This panel will allow you to change settings and information for any user on your board. To use it, simply enter a username into the text field and click Submit.
This panel will allow you to edit any part of a user's profile. The panel is identical to the standard profile panel, with a few extra settings at the end. For more information, see 4.7 Profile Settings.
At the bottom of this panel there are Special admin-only fields that are not part of the standard profile settings.
User is Active - You may deactivate/activate a user. This is most often used to activate a user who is having trouble with email activation. A user may not log in while their account is inactive.
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Delete this user? - To delete a user, check this box and click Submit. A deleted user's posts are converted to Anonymous posts with their old username as the name specified.