Although everybody calls it a 0% chance, it's enough for just one man to get the key to a big wallet like Satoshi's
Satoshi's wallet is not one address or public key. It is tens of thousands of individual keys because he did not reuse them. Being able to find a private key that he used would only let you spend 50 Bitcoin.
I understand the chances are very close to 0
You don't seem to understand how close to 0 it is.
It is literally impossible to have the probability of collision be exactly 0 because that would require an infinite search space which is literally impossible (would require infinite matter and infinite energy which do not exist).
but what if it happens just ONCE with a huge wallet? Is there any way this could be prevented?
There is no way to prevent it because whoever produced the collision would have an equally legitimate claim to the Bitcoin.
It is far more likely that if there were a collision that it was the result of a bad implementation of the RNG and of the crypto library used in general.