Add a .htaccess file to redirect HTTPS to HTTP, and to redirect requests for the robots.txt file to one that dissalows HTTPS crawling:
# Redirect HTTPS to HTTP
RewriteCond %{HTTP:X-Forwarded-Proto} =https
RewriteRule ^(.*)$ http://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
RewriteCond %{SERVER_PORT} ^443$
RewriteRule ^robots.txt$ robots_https.txt
Then add a robots_https.txt with this in it:
User-agent: *
Disallow: /
