Simply edit the following language string to adjust the error message.
IN: components/com_comprofiler/plugin/language/default_language/default_language.php
ON: Line 913
FROM:
[code:1]
define('_UE_USERNAME_ALREADY_EXISTS',"The username '%s' is already registered: please choose another one."«»);
[/code:1]
TO:
[code:1]
define('_UE_USERNAME_ALREADY_EXISTS',"The username is already registered: please choose another one."«»);
[/code:1]
Don't understand how it will make any difference as the user typed admin and it clearly was denied, so all need be is look back at the username field as to what the typed. Such error is typical for any site as far as I am aware.
Perhaps more intuitive passwords are in order. Using some of the other validation methods for the username field would be a better approach such as requiring at least 6 characters, 1 letter, 1 number, etc...