Hi Tomas! You know I think that plagiarism in this case is like unsportsmanlike behavior during competitions, like doping.
Downwoting maybe one of the ways to punish for plagiarism. Also one way which I see it is to disallow to participate in your contests to people who were seen in plagiarism.
As you know there is chetah bot which controls or makes the appearance of plagiarism, but is is works with post, not comments if I'm not wrong. And as I know there are many questions to it's work. Maybe if you work with Python or Java you can create such unplagiarism bot or library to control plagiarism in your contests. I think all steemians will be thankful to you. Good luck!