Identify hosts that you don't want on your site. Try it with:

$ curl 'http://v1.nastyhosts.com/52.30.0.1'

{  
  "status":200,
  "asn":{  
    "asn":"16509",
    "name":"Amazon.com, Inc."
  },
  "country":{  
    "code":"US",
    "country":"United States"
  },
  "hostnames":[  
    "ec2-52-30-0-1.eu-west-1.compute.amazonaws.com"
  ],
  "suggestion":"deny"
}

What is nasty?

NastyHosts.com was designed for web applications and tries to detect all IP addresses from VPS clouds, VPNs and known spammy networks. It CANNOT be used to test mail servers.