they have it as after the competition ends the average vote is taken and your deviation from that score is the important part of how fairly you voted.  To offset that, there are "jurors" who have a massive and unchanging karma and are those who are more experienced in what is good and what is bad (this is in terms of photoshopping things but could be transfered to photography).  For every time you vote within a cerain range your karma increases, if you vote outside the range from the average your karma goes down...
Obviously there isnt a time limit here for votes, but maybe if you put a time limit of 3 days to work out the karma then it might work - as after that time it is rare to get a comment on an image unless it is spectacularly good and people share the link to it...

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography