prevent CTRL+V And Shift+Insert in textbox c#

To get rid of the Context Menu of a TextBox, and disable all short-cuts: set the 'ShortcutsEnabled property of the TextBox to 'false.

Or, you can disable the entire context-menu for a TextBox by assigning an "empty" ContextMenu to the TextBox 'ContextMenu property:

private void YourMainForm_Load(object sender, EventArgs e)
{
    textBox1.ContextMenu = new ContextMenu();
}

If you need a custom Context Menu, you can, of course, add a ContextMenu to your Project and set it to be the ContextMenu of the TextBox.

Here's one way you could block keyboard paste; handling Shift/Insert is left for you to code:

// example of blocking Control-v/V

private bool isControlKeyDown = false;

private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
    if (ModifierKeys == Keys.Control) isControlKeyDown = true;

    if (isControlKeyDown && e.KeyCode == Keys.V)
    {
        e.SuppressKeyPress = true;
    }
}

private void textBox1_KeyUp(object sender, KeyEventArgs e)
{
    isControlKeyDown = ModifierKeys == Keys.Control;
}