simplest way to keep someone from accessing a page unless they are directed there via link?

I wrote a page that allows people to vote "like" or "dislike" a page. I am ATTEMPTING to limit their voting by keeping track of their IP address. I made it so that one could vote only once per IP address.
However, after checking the database today I found an IP voting twice per company, changing the IP and then voting twice for another company. Here is a sample from the database.
 
ID Vote Company IP StateID 213 2 1 38.127.197.109 10 214 1 1 38.127.197.109 10 215 2 1 38.127.197.100 38 216 1 1 38.127.197.100 38 217 2 1 38.127.197.110 45 218 1 1 38.127.197.110 45 219 1 1 38.127.197.110 17 220 2 1 38.127.197.110 17 221 2 1 38.127.197.110 3 222 1 1 38.127.197.110 3  As you can see the IP changes slightly and then two votes are recorded per company. I think they are simply running the page that has the code on it. What is the simplest way to keep someone from ac ...