I do not know how to report that someone is cheating, or if you did would Microsoft care.
I watched my results on the leader board on the event that finished on saturday last.
I finished the event on Friday evening and was in 6th position. Saturday at 10am iwas 8th, midday 9th and 1.30pm still 9th. At 2pm when the event finished, half an hour later, I had gone to 37000th, with the top 100 having amazing times.
cheers
terry