If you encounter someone using a cheat or hack, please submit a support ticket at
https://www.hirezstudios.com/submit-support-ticket/. Include any and all relevant information. If you have evidence, that's really helpful for verifying the evidence. However, if a name or match information is all you have, that at least gives the support team somewhere to start looking. The in-game report feature also exists and is triggered if enough reports are sent against a specific player.