The last time we did a deposit bonus giveaway it was for Golden Tickets and in exactly the format you suggested and ran for 48hrs each time. Unfortunately, one individual started depositing and withdrawing the same 2 BTC constantly for 48hrs every time we ran it. We think this is a lot fairer on everyone and a lot less attractive for high rollers to abuse.
There will always be some people who will use your giveaways in a bad manner. I think it was you who said that you caught people with thousands of fake accounts (not just one person with thousand but total) which means there were thousands of accounts that claimed free faucet to get money. This is like what 2 satoshi? How much can you make from this? However one thing is really obvious, if there is a way that any website could be abused for money, there will be people who will abuse it.
This is why I think it is obvious that there will always be people like that and we can't do anything about it. Not saying we should do nothing about it but it is really not simple to force them not to do it, it is easier to stop them after they did it.