Disclaimer: I'm not highly experienced in user input classes, but here's my input.
CTRL and ALT are examples of modifier keys. That is to say, they modify other (non-modifier) keys to create a key combination. Your UI is likely only able to pick up a complete key combination. For example:
private void keyPressed(object sender, PreviewKeyDownEventArgs e)
{
e.KeyCode == Key.A; // True (pressed A)
e.KeyCode == Key.Control; // False (no key pressed)
e.Modifiers == Keys.Control; // True (user is pressing the modifier CTRL)
e.KeyCode == Key.A && e.Modifiers == Keys.Control; (pressed key A with modifier CTRL)
}
As for disabling the key, you could just catch e.Modifiers:
private void ignoreCtrl(object sender, PreviewKeyDownArgs e)
{
if (e.Modifiers != Keys.Control) { /* Pass to handler */ }
else { /* Discard */ }
}
Again, I'm not experienced in your particular framework but this would be my guess. I used the following SO sources: