bool isChecked =false;
    private void radioButton1_CheckedChanged(object sender, EventArgs e)
    {
        isChecked = radioButton1.Checked;
    }

    private void radioButton1_Click(object sender, EventArgs e)
    {
        if (radioButton1.Checked && !isChecked)
            radioButton1.Checked = false;
        else
        {
            radioButton1.Checked = true;
            isChecked = false;
        }
    }