making anonymous poll - voting only once

[JayKay]

So I have made an anonymous poll system but I would like to limit the possibility to vote only once from a single pc.

 

I think that I could store and check 3 things regarding a user that has voted:

1) $_SERVER['HTTP_CLIENT_IP']."!".$_SERVER['HTTP_X_FORWARDED_FOR']."!".$_SERVER['REMOTE_ADDR'];

2) HTTP cookie

3) Flash cookie LSO (because this contrary to the HTTP cookei can be detected in multiple browsers( (only if Flash is installed)))

 

But there can be a simple situation - a simple user votes one time from Firefox and after a time his IP changes (for example he uses some Mobile internet where dynamic addressing is used) . Then for some reason he wants to use Internet Explorer where Flash is not installed. In result - it is possible to vote twice.

 

Can anyone please suggest something regarding this?

 

I know there is not 100% bulletproof solution - I just want to minimise the possibility to vote more than once.

 

Also I am thinking about using EverCookie(undeletable cookie) but that could cause a lot of other problems.