The incentive to cheat is quite weak. The jam takes place every week and the value of winning one isn't that high. If you would look at the hall of fame, the names change quite a bit, so there doesn't seem to be anyone who is going out of their way to keep winning at all cost. Last year we had a rating "time", if developed within 3 hours, you'd get 5 stars, more time, lower rating. This imho was a good system, but sometimes people would rate games made in less than 3 hours with less than 5 stars, which was confusing and even upsetting to some developers. So instead, this year we went back to the original system that we had at the start. We will do a survey closer to the end of the year to see if the community is happy with this system to determine how we should go forward like we did last year (people voted for the current way of choosing the winner). Overall, the biggest winners are those who focus on their craft and improve as developers. You can see that looking at devs who participate regularly and how much their work improves! :)
If you have suggestions how to improve the jam you are more than welcome to share your ideas!