If you want a complaint to have more weight than "My mate told me this geezer's dodgy", then I recommend undertaking some addition research to support the accusation.

For example;

1. Identify all of the winners of their Engagement challenges and other contests. Are any users awarded more prizes than others?
2. Categorise these users. Country of origin, who introduced them and when they joined are often good indicators. You could also check the chain of introductions to see if they're friends of friends. Have they won other challenges? Do they only enter contests or is there something that makes the profile more personal?
3. Identify when this community was first selected for engagement challenges. It's common for the less honest to have accounts originate around the time that they receive some element of responsibility.

In my opinion, this is the minimum.

At first glance, the 3 prize winners identified for a steemcurator vote look fine.

Then you see that the 2nd place was introduced by the contest organiser who selected the winners.

If this happens consistently, it's more than simply coincidence. It's also common practise for their friends to win 2nd and 3rd, rather than 1st. It's more subtle that way.

Then, you'll notice that the first booming nomination was also introduced by the contest organiser.

Like I suggested, it's more subtle to award secondary prizes.

So a steemcurator vote and a booming vote go to the organiser's mates.

This light touch investigation gives some actual evidence to support your accusation. Without it, your complaint reads like: "I didn't win a contest that had 163 entries, they must be cheating. My mate said they're cheating."

Even if they probably are cheating.