using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string payload = @"

           {

               UserName: ""value"",

               Field1: value1,

               Field2: value2,

               ""UserAccount"": value3

           }";

        Console.WriteLine(payload);

        payload = Regex.Replace(payload, "UserName.*,|UserAccount.*,|Country.*,|State.*,|City.*,", string.Empty, RegexOptions.CultureInvariant);

        Console.WriteLine(payload);

    }

}