What is the best/easiest/most efficient way to track specific visitors to your website?

Here is what I am wanting to do:

Once someone opts in (with name and email), that person will receive an email. From that email, they will be directed to a webpage with a series of videos. At the end of the video, they will be given a limited-time ONLY offer that they can take advantage of within a set period. If the time expires, I need a way of excluding that person from taking advantage of that offer.

Using the name &/or email to track this is makes it too easy for the person to cheat the system, so to speak. The only way I can think of is using the IP Address along with the name & email. This still makes it pretty easy to cheat the system but I think that the majority of people (at least in my target audience) won't know how to do so.

1) Is there a better way with such limited info?
2) If not, how can I easily record and track the IP Address?

I know php enough to (kinda) get by and I am assuming that is how to pull this off but am unsure of just how to do it. Any help is appreciated. Thanks!

Use Google Analytics, bing webmaster tool etc. By using their rich features you will be able to get the exact idea about the visitors.