Disallow Robots

That is close. robots.txt is a text file, so you don't need to send HTML tags in the response. You can use \r\n to insert a new line:

when HTTP_REQUEST {
   if {[string tolower [HTTP::path]] eq "/robots.txt"}{
      HTTP::respond 200 content "User-agent: *\r\nDisallow: /"
   }
}

It looks like Google has a robots.txt analyzer you might be able to use to verify your rule:

How do I check that my robots.txt file is working as expected? (Click here)

Aaron