How to stop hook_menu item being called
No, you can't, see the following question/answer: https://drupal.stackexchange.com/a/33493/31

If you only want to allow certain arguments, you need to validate the argument and return MENU_NOT_FOUND. Or you can use a named argument, e.g. cars/%brand, which will automatically call the brand_load() function, pass the value of the parameter to it and if that function returns FALSE, then it's automatically returned as 404. This is used for node/%node, user/%user and so on.