You need to read more of this thread to understand the scoring. The scores are weighted towards the end of the round to stop this. A hopper doing what you describe would get little or nothing from most rounds.
It doesn't stop it its usually fairly easy to tell when there are multiple hours left in a block. You can come back in the last 30 minutes and get most of the reward. I don't do so but use the longer blocks to mess around/tweak hardware.