Clint that might be the case with some developers and of course there will always be some one who wants to rig the process. Sad but true... But I can honestly say that 1stSpin has caught one of the TA users cheating in some of his recent contests. You can follow some of the threads there. I have never met 1stSpin nor his company nor do I even work with his company or any other item you can think of. Nor was even I in the running for any contest either (kinda hard if one doesn't enter! LOL..since I know I s*ck!) Thus hopefully the TA moderators can be alerted and do whatever is necessary, If anything, to such person. Also, found it odd that when the person was caught not a peep from such person in any of the threads. We do also know such person has an accomplice as well. Just thought I pass this info along...
Instead of guidelines, why don't you ensure the winning prize? One way to ensure the winning prize is to force developers to send an e-mail gift card to TA (before starting the contest) and in the end of the contest TA forwards the prize to the winner. If the developer won't announce a winner in time, a random competitor gets it. It would be fool-proof and it would attract more competitors.
While contests are a good way to spark some interest, it is quite annoying that so many devs scam on this, so a legit developer doesn't really want to try it out due to the bad experiences users have made before. I have read in the rules that TA staff would act as a escrow service. Is that possible? Seems to me it would be quite some extra work for you guys. If so, please let me know! Best