PocketMagic

 
  @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
    	if (!b.isChecked())
    		return super.onKeyDown(keyCode, event);
 
        if (keyCode == KeyEvent.KEYCODE_MENU) {
            return true;
 
        }else return super.onKeyDown(keyCode, event);
    } 
 

If the checkbox is on, the MENU long press will be overridden and the keyboard won't open. A side effect is that also short MENU presses won't work anymore (like when trying to open a predefined application menu).

Source code: OverridingMenu