This was tested in phpBB3 and it works.  Specially you can use this for spammers who come with names that are vulgar or userIDs that you do not want showing in your list of users.

Go to Users&Groups / Prune Users
Click the “Find a Member” link
Checkmark the users to delete.
Click “Select marked”
Select Delete
Submit.

Source: Brf from phpBB community.