User-agent: * 
crawl-delay: 500

ErrorDocument 503 "Site temporarily disabled for crawling"
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} ^.*(bot|crawl|spider|80legs).*$ [NC]
RewriteCond %{REQUEST_URI} !^/robots\.txt$
RewriteRule .* - [R=503,L]