The Shoutcast Directory Tester user agent is what shoutcast.com uses to check that you are streaming something on your radio. They do this every time you connect to your radio and before they list you. You can stop it by changing your encoder configuration to disable making your radio listing public. This would stop you being listed on shoutcast.com and probably cause you to have less listeners. Being listed on shoutcast.com is a great way to promote your radio station in our opinion and a major reason why people choose a Shoutcast Server over an Icecast Server.