AFAIK unlike storage coins (eg. sia or storj), it's not being used for anything, it's just "burned". Proof of work burns compute cycles. Proof of storage burns storage space.
What prevents this?
Once a farmer wins a block it's theirs, the same way it works with bitcoin. The reward is tied to a public key, not the original storage. Just like in bitcoin once a miner wins a block they don't need to continuously prove that they have x amount of hashing power in the future.
You can learn more about the details of the blockchain and algorithm here: https://www.chia.net/greenpaper/