Sorry I don’t have a useful answer, I just want to share my experience.
About 8 years ago I worked for a site that ran online polls. We once got hacked by some bots voting on some of our questions several hundred votes a minute.
I had to implement some emergency checks: IP address, cookies, and I really don’t remember what else.
At the end of the day we decided to take the polls down. Those damn robots just didn’t care. IP adresses were spoofed, cookies were being deleted, etc.
If you really really need the polls to be unhackeable I don’t see any other way that requiring registration and using captchas to avoid bots signing up for new accounts.
And the sad thing is that this was just an entertainment site with polls on what’s your favorite color and things like that.
The only thing we didn’t tried was using captchas because they didn’t exist at the time. That might have reduced non-human cheating a good deal.
Don’t take this as any kind of expert advice on the matter, because that was the only time I had anything to do with online polls, but I remembered my story and wanted to share.