This would be the exact line:
https://github.com/q2a/question2answer/blob/master/qa-include/qa-lang-misc.php#L34
Of course, if you're using your own language files you will need to change that same file but in the qa-lang directory.
Alternatively, just create a custom directory under qa-lang and copy paste that file and removed the unneeded translations. EG, create file qa-lang/custom/qa-lang-misc.php with the content:
<?php
return array(
'captcha_login_fix' => 'To avoid this verification in future, please ^1log in^2 or ^3register^4.',
);