Maybe we should simply stop to pay the abusive ones indeed!
I strongly suggested you to revise the rules.
You should set a minimum account age / activity requirement to enter the giveaway.
Also, you should specify suspicious account will not be paid.
For example, the Primedice giveaway is only for accounts with 50+ activity, Stunna will not pay suspicious accounts.
To avoid abuse, you must have at least 50 forum activity. We reserve the right to not fund any accounts we believe to be duplicates of individual users.
If you keep crediting those accounts, it would just encourage people to further abuse your giveaway, and it is extremely easy with good return.
Step 1: Create 20 accounts (easy, no email confirmation is needed)
Step 2: Make a stupid post in newbie section for each account (eg. OMG, why I can't post in XXX board)
Step 3: Watch some tv or walk the dog.
Step 4: Send BTC to 20 different temp addresses, and play your game once using each address.
Step 5: Claim the giveaway 20 times (0.005 * 20 = 0.1 BTC, a lot better than mining)
Step 6: Have a nice sleep to prepare for abusing the giveaway again the next day.