This post describes what the rules are for a contest violation, but not how to report a contest violation. This is how you report cheating:

• After the contest, go to the contest homepage. View all the submissions by clicking "More" under the "Ranking" table.
• If you suspect a user cheats by using an explicitly disallowed library function or copying code from other resources, please report it by clicking on the "Report Cheating" toggle under the suspicious solution.