Similar topic already discussed at
https://bitcointalk.org/index.php?topic=5072435.0;allThe easiest/laziest solution would be :
1. Only use blockchain as entropy source when the cost to manipulate blockchain is higher than it's "reward"
2. Use many blocks in a sequence as entropy source since it's difficult to manipulate
n blocks in a sequence.
So basically there's no solution, because Bitcoin blocks come too rarely and no one would wait more than a few seconds for a random number. My guess is that we'll first see a bullet-proof random number generator on Ethereum.