Preprocess Comment before Akismet

In my functions.php file, I’m trying to preprocess comments before Akismet does. It doesn’t appear to be working.

function comment_spam($commentdata) {
$email_address = trim( strtolower($commentdata['comment_author_email']) );
if ( $email_address == 'banned@home.com' ) {
wp_die('Sorry, your email address has been banned from posting comments.', '', array('response' => 403));
}
}
add_filter( 'preprocess_comment' , 'comment_spam', 1 );

Looking at the plugin source code, it looks like Akismet also uses a priority of 1 on preprocess_comment filter – and is getting fired off before mine.

What can I do to fix this?