Ideally, if you want to report that some users have cheated, or if you want to appeal against having been caught for plagiarism, you should only send an email to help@codechef.com , and wait for a few weeks to get a reply.
But despite this instruction, if you do feel that you need to say something publicly, you can use this topic. Any other new topic/comment that you start related to this will be closed, and you will also risk getting suspended.