Bitcoin is a gossip network. The node's job is to listen for information from its peers and relay information to its peers. Knots simply doesn't acknowledge spam transactions for the duration of time that the transaction hasn't been mined into a block. Knots does extremely little to slow down the propagation of spam transactions across the network and would have to achieve near totality of the network to do so, and even then, it cannot totally stop a miner from mining a spam transaction that was delivered to them oit-of-band (not via nodes).