There appears to be a filter which does exactly what you request It is in the filter box on the lower right hand side of the challenge moderating page. I have just run it and it brings to the top those challenges I haven’t voted on yet. The four choices in the filter box are:
* Show all
* Show those I haven't voted on
* Show those where I have voted against the crowd
* Show those with at least two positive and negative votes
Hope this helps