How do I prevent a control from receiving focus when it receives a mouseclick

You can set the control’s Enabled property to false. This will prevent clicking but also gives the disabled look.

There is a ControlStyles.Selectable flag that determines whether the control can get focus or not. But it does not appear to affect some controls such as TextBox. But you can prevent the TextBox from getting focus by overriding its WndProc method and ignoring the mouse down.

public class MyTextBox : TextBox
{
	const int WM_LBUTTONDOWN = 0x0201;
  
	protected override void WndProc(ref System.Windows.Forms.Message m)
	{
		if(m.Msg == WM_LBUTTONDOWN)
			return;
		base.WndProc(ref m);
	}
}