If you are the one who registered his server here you can unregister the server yourself. Just login through the EDIT Tab and delete the registration of the server.
But if you are for example the new owner of a server and the previous owner had already registered his (now your) server here and you want to have the registration get deleted, for example to make a registration yourself or if you dont want to have your server be registered here then you can claim the ownership of the registration.
To do that go to the corresponding server, click on the EDIT Tab and click on Claim Ownership, follow then the instructions.