Assuming you have already logged into phpBB. Click on the Users and Groups link found on the main menu.

Then click on the Disallow usernames link found on the bottom left side of your screen, under user security.

Enter the name you wish to disallow in the screen shown below and click on submit button.

That’s it, the name you choose to disallow can no longer be used in the forums. Now go and remove the name you choose from the list, so it can be used once again.

In the remove a disallowed username,  click the submit button,  the name is now allowed to be used again.
That’s it, you now know how to disallow usernames in phpBB.