Cancel Key press event

Use the Handled Property

e.Handled = true;

Example from MSDN: link

// Boolean flag used to determine when a character other than a number is entered.
private bool nonNumberEntered = false;

// Handle the KeyDown event to determine the type of character entered into the control.
private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
{
    // Initialize the flag to false.
    nonNumberEntered = false;

    // Determine whether the keystroke is a number from the top of the keyboard.
    if (e.KeyCode < Keys.D0 || e.KeyCode > Keys.D9)
    {
        // Determine whether the keystroke is a number from the keypad.
        if (e.KeyCode < Keys.NumPad0 || e.KeyCode > Keys.NumPad9)
        {
            // Determine whether the keystroke is a backspace.
            if(e.KeyCode != Keys.Back)
            {
                // A non-numerical keystroke was pressed.
                // Set the flag to true and evaluate in KeyPress event.
                nonNumberEntered = true;
            }
        }
    }
    //If shift key was pressed, it's not a number.
    if (Control.ModifierKeys == Keys.Shift) {
        nonNumberEntered = true;
    }
}

// This event occurs after the KeyDown event and can be used to prevent
// characters from entering the control.
private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
    // Check for the flag being set in the KeyDown event.
    if (nonNumberEntered == true)
    {
        // Stop the character from being entered into the control since it is non-numerical.
        e.Handled = true;
    }
}

You may use keypress event as below. use e.Handled to true to cancel user input

    private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
    {
        if (!Char.IsDigit(e.KeyChar)) e.Handled = true;
    }

Create a string with the characters you what to allow the user to enter.

Use KeyDown or KeyUp to handle Special keys

private void tbN1_KeyPress(object sender, KeyPressEventArgs e)
{
    String sKeys = "1234567890ABCDEF";
    if (!sKeys.Contains(e.KeyChar.ToString().ToUpper()))
        e.Handled = true;
}