I wanted to write that it's the simplest line of code to make an 'alert' window open : Ding! have you voted? on closing a page, or on opening, BUT I'm actually not so sure FireFox will process such codes, it sounds like a Microsoft thing using IE to access Windows, hens all the security issues.

Those Ding alert windows are immensely irritating though! 
They always shock the shits out of me!

And "pop-ups" are stopped by FF, so you'd have to tell every one to 'allow popups' which is in its self a breach and not really welcome!

But then I'm also not a programmer either.

After thought :  you could write into the pages that all buttons and links that exit the page will prompt "Please vote!" if and only if a viewer has not voted, wouldn't work (??) for 'closing' the browser or the page, but it would work if you want to 'stay within' the sites pages.
We would then quite quickly vote if only to not have the 'Ding' violate the blissful experience of reviewing!