[编辑 – 添加代码示例]

richTextBox.TabStop = false;
richTextBox.ReadOnly = true;
richTextBox.BackColor = Color.DimGray;
richTextBox.Cursor = Cursors.Arrow;
richTextBox.Enter += richTextBox_Enter;

private void richTextBox_Enter(object sender,EventArgs e)
{
    // you need to set the focus somewhere else. Eg a label.
    SomeOtherControl.Focus();
}

或者作为扩展方法(我意识到你不必将它放在readonly中,因为Enter事件捕获任何输入)：

public static class MyExtensions
{
    public static void Disable( this Control control,Control focusTarget )
    {
        control.TabStop = false;
        control.BackColor = Color.DimGray;
        control.Cursor = Cursors.Arrow;
        control.Enter += delegate { focusTarget.Focus(); };
    }
}