You can set a timeout for submissions per user so you can make sure a user doesn\’t submit a form too often. This is especially useful if you use NinjaForms for any kind of votings for example when you create a photo contest with Ninja Forms Multimedia Fields.
Of course a cookie is not as save as validating logged in users but it is fast and simple and you don\’t force your visitors to create an account.
Check out the image voting demo here