First action is to contact any form of customer support of the said site and tell them what happened and show them what you've got during your observation. Wait for them to respond back, see if they will help you or advise you something else. If they insisting that there is no cheating happening, gather more information. If they stopped cheating on you, then it's up to you if you want to pursue it for other users or just continue playing. Legal works are kind of expensive nowadays, and not worth the hassle if we are talking losses of a 50 to 100 bucks. Just go to another site and use anonymous accounts and bad mouth the said site.