I'm developing a website to find out the best politician of my state.In my website visitors can vote for their favorite politician.
I want to restrict the users adding more than one vote to a politician and also wants to restrict a user vote for more than one politician (A user can give one vote to any one politician).
How can I trace unique users? I'm not interested in adding a registration/logon method which will reduce the user interaction. I tried to trace unique IP address, but website can be accessed through proxy servers and that method will not work.  
Is there any way to trace users?
For this task, you have to user registration/login functionality. Otherwise, you will get incorrect data.
My suggested workflow:

User registers on site with his email id.
Confirmation mail goes to user.
User clicks on confirm link and proves that he is genuine.
After that he votes.
His vote gets recorded against his email id.
unique user identification for a website