This.
Let's say your user has IE, Chrome, and Firefox all installed on one machine. They can post three times, regardless of what tracking cookies you employ. They can open a new incognito window and post again from there. If they're already changing their IP addresses in order to avoid getting caught, there's little you can do. Tracking cookies help, but they're far from bulletproof. And MAC addresses never make it that far up the network... and even if they did, the users could change their MAC address, too.