First of all the menu is called context menu. Second ALT+SPACE is handled by OS i would suggest to use some other key combination however if you still want to do the same easy way is to set control box property of the form to False.
public partial class Form1 : Form
{
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool GetCaretPos(out Point lpPoint);
public Form1()
{
InitializeComponent();
}
private void richTextBox1_KeyPress(object sender, KeyPressEventArgs e)
{
}
private void rtb1_KeyDown(object sender, KeyEventArgs e)
{
Point p= new Point();
if (e.Shift && e.Control)
{
GetCaretPos(out p);
cmenu.Show(rtb1, p);
}
}