Django guests vote only once.

I'm new to Django, but I'm working on a tutorial on the Django site to create a survey.

What is the best way to ensure that guests (without registering / logging in) can vote only once in a poll?

• IP (Do not want IP, because people who use the network can vote only once).
• Cookie (The user can delete the cookie, but is best suited).
• Session (if the user closes the browser, the session will change).

I suppose Cookie would be the best approach, but is there a better way for Django?