Make every comment go to the spam folder

I don’t know of a way the default comment status can be changed, but there’s an action 'comment_post' that runs after comments are inserted that you could easily hook to set every new comment to have the ‘spam’ status:

function set_new_comment_to_spam($commentId) {
    wp_set_comment_status($commentId, 'spam');
}

add_action('comment_post', 'set_new_comment_to_spam', 10, 1);

Add that to your functions.php