I am not going to mention any specific accounts, but I will say that where there is money involved there are always going to be cheaters hanging around. Look at the numerous amount of bounty cheaters, sig campaign cheaters breaking the rules, and the amount of scammers that have found this forum. It's a never ending cycle of greed, so it would not surprise me 1 bit to know that people created accounts just to cheat and participate in contests with cash prizes with their alts.
Absolutely and I don't think forum can do anything about that except the use of trust system to justify them, but even as that they don't still care about that and they would definitely find away out to always continue cheating.
Now, what should be the modalities to tackle this issues? Well, to me tagging account would help, but rather I will suggest there should be a place to records profile that has cheated, when contest or new sig launched manager should be able to check through that list to justify if such person can be accepted or not. Though the trusting system already