You could duplicate the quip.rampart hook and change this:
$email = $fields['email'];
$rptSpammerErrorMessage = $modx->getOption('rptSpammerErrorMessage',$scriptProperties,'Your account has been banned as a spammer. Sorry.');
$response = $rampart->check('',$email);
to this:
$email = $fields['email'];
$username = $files['email']; //added
$rptSpammerErrorMessage = $modx->getOption('rptSpammerErrorMessage',$scriptProperties,'Your account has been banned as a spammer. Sorry.');
$response = $rampart->check($username,$email); // modified
Then specify that duplicate hook in the tag instead of the original hook. This may or may not make much difference as spammers change their usernames regularly. You could also write your own additional hook and keep a local file with a list of spammer usernames to check.
You could also make that change in the existing hook, but then it would be overwritten if you upgraded Rampart.