Identify if same user browser - also when incognito/private browsing

I have created a simple poll web application. It has some simple funtionality - the landing page is just the form for a new poll with some options and a create poll button. Then I have the poll page where people can vote on the different options and see results.

I don't want users to sign up to submit a vote, but I want to restrict users from voting several times. My solution to this has been to add Google Analytics on page, and then I can via the _ga cookie identify the individual browser. But, someone could just go into incognito mode and the temporary cookie would be different for each time a new browser windows was opened and users could just vote several times.

How can detect the same browser, also when the person opens an incognito browser?