To configure PURGE add this lines into your VCL_RECV:

vcl 4.0;
import std;

sub vcl_recv {

  # Allow banning  
  if (req.method == "BAN") {
     # Same ACL check as above:
     if (!client.ip ~ purge) {
        return(synth(405, "This IP is not allowed to send BAN requests."));
     }
	 ban("req.http.host == " + req.http.host + " && req.url == " + req.url);
     # Throw a synthetic page so the request won't go to the backend.
     return(synth(200, "Ban added"));
  }

}

To test it:

$ curl -v -k -H "host: www.mydomain.com" -X BAN http://varnish_server_ip/