How to disable it in my tools

People seem to be bypassing my authentication system using this and i am wondering if there's a way to detect it being used...I've tried

if (Process.GetProcessesByName("fiddler").Length != 0)
        {
            MessageBox.Show("Internal Error");
            Close();
        }
But it doesn't seem to detect it....if anyone could help that would be GREAT..Thx..!B