Fair quiz without registration, is it possible in webapp?

Question

I'm going to make simple quiz form on site.

Are there ways to make it fair, that nobody vote up multiple times.

There are some ways to identify uniqueness of vote with their lacks:

• Cookies - the simplest and easy to eliminate
  
• IP - not stable,because of DHCP and local networks
  
• Captcha or mail registration - not user friendly and complicate the process.

What is your experience for fair and user-friendly quiz?