I think that you should be able to disable it in the language file, moodle.php, by changing the lines
$string['enrolmentkeyhint'] = 'That enrolment key was incorrect, please try again<br />
(Here\'s a hint - it starts with \'$a\')';
to
$string['enrolmentkeyhint'] = 'That enrolment key was incorrect, please try again<br />';