User's IP Validation

Hi people, I would like to know the best way to perform some kind of validation for an application that I've written. I have a system that ask through an HTML Form some questions to users. I use some cookies to save some information from the user side. However, I would like to implement some code in PHP that would let me limit to 1 the number of times that the page with the questions was executed. I mean, the user fills the HTML's Form, then send it through an HTML Button, then PHP receives this informations and send an Email containing the replies to the questions. I would like to limit to one, the times one single user is able to execute this form. I thought getting the IP Address, then doing some kind of validation with it. However I don't know if using cookies is the best idea. I don't have access to a DataBase for this. So I thought might be a good idea write to a file in the server the IP, then perform some if to know if the user already replied the form. As far as I don't know which is the best way to code this, I felt free to ask you guys. Thanks a lot. Juan