we are trying to make a website all based on people voting for certain medias.

The people would have to go on a page, click on a picture/link/whatever that would call a .php page (using AJAX) that would then:
1- verify with the person' I.P address if he as already voted today
2- if not, then add the vote to the database
3- if yes, return a message like "you already voted..."

without even reloading the actual page

The question would be:
if there any other way then with the I.P to control spam votes?

But our main concern is actually that someone could create an apps/robot that would "simulate" a user with a different I.P address and spam our website easily!

So how to make sure if the "click" is legit 100%???