In a discussion about making transaction fees mandatory, Satoshi Nakamoto proposed 16 years ago limiting the amount of free transaction space before charging, but never reducing that allowance to zero. The transaction fee is what stops people from spamming the network.
The same keeps people from spamming SN.
The fee does the heavy lifting, but it is not the whole story. Bitcoin has never actually removed Satoshi's free allowance, and spam still does not win because relay policy drops too-small or too-dusty transactions before they ever make it to a block. So the fee is the price signal, and relay is the spam filter.
On SN it is the same two layers. The stack cost is the obvious one, but the quieter filter is that an account and a posting history have effort behind them. Both matter, and neither is foolproof, which is why the answer to spam is usually "raise the cost a little" rather than "one perfect fix".
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