I would suggest to let participants to post authentication in the thread (or any proof that they're the one who filled up the form), as anyone can fill up the form and use anyone's username (profile) to claim the $10.
BC.GAME says rewards will be sent via PM, meaning anybody registering using another person's username will lose out on the rewards...
And like you pointed out, threat of abuse exists as this is open to all as new accounts can be created just to try and claim rewards several times.
Users will receive a $10 claimable bonus code with no wager requirements in their Bitcointalk PMs once we've reached 100 eligible responses.