It is indeed one of the hardest things to do.
People do underestimate all the time that's involved with running a contest like yours. Once people do see that some SBD or Steem could be won, they can get very creative with finding ways to increase their chances.
We did discover the same issue with our World Cup Contest!
Keep it going on @nanzo-scoop. We really appreciate these kinds of contest which does try to increase the interaction on the Steem Blockchain!

Cheers,
SBC