If they want to do it every 3 blocks, they might want to make sure the transactions won't have trouble getting through in case 3 blocks are mined rather quickly after each other. Or, they just had 300 BTC handy that they didn't want to split up. Or.... ??
If I sat there using online banking to transfer money from one account to another over and over - my bank would put a hold on my account. I think there should be a way to prevent this. This one person is unlikely to have any effect but if the bot-net dickheads get hold of it...
There is no way to prevent it besides changing the fee structure. 300 BTC is enough that it could be sent in every block without a fee.