They already are stopped. Bitcoin Core enforces a dust limit... 546 sats for P2PKH. Any tx below that gets dropped by default. So yeah, we’ve been filtering spam for years. This means that if a spammer tries to send 1 satoshi, it won’t propagate across the network unless miners are directly accepting and mining it themselves. The dust limit was introduced specifically to combat spam, by making it 546 times more expensive to flood the network with meaningless outputs.