This tutorial assumes you've already logged in to phpBB
Now let's learn how to disallow usernames
1) Click Users and Groups
 
2) Click Disallow names
 
3) Enter a name you wish to disallow here

4) Click Submit
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That's it! Â The name StoreHater can no longer be used in the forums

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Now let's remove it from the list so it can be used once again. Â Scroll down

5) In the Remove a Disallowed username section, click Submit
 
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The name is now allowed to be used again

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This is the end of the tutorial. Â You now know how to disallow usernames in phpBB