Disable a TeamCity agent through the REST API in C# http://devnet.jetbrains.com/message/5462246#5462246

TeamCityDisableAgent.cs

string address = "http://<TEAMCITY>/httpAuth/app/rest/agents/id:1/enabled";
byte[] data = Encoding.UTF8.GetBytes("false");
 
using (var client = new System.Net.WebClient())
{
  client.UseDefaultCredentials = true;
  client.Credentials = new NetworkCredential("<USER>", "<PASSWORD>");
  byte[] response = client.UploadData(address, "PUT", data);
}