Hi Andrea! Howdy!

Great question! I will later do a more detailed answer but in nutshell:

• Can a non-validator Node reject blocks?

In Ethereum PoS, a non-validating node (i.e. with no 32ETH), will still be able to reinforce the consensus mechanism because he will still be verifying all the blocks and won’t be able to accept blocks that don’t follow the consensus.

These nodes can also help the network recovering in case of attack (as they keep the entire ledger and will help with healing the network). They contribute to a more robust a censorship-resistant network and they do a great job helping light clients (like desktop and mobile wallets) to synchronize their transactions.