What's keeping attackers from spamming the transaction pool with low gas tx to throttle ethereum?

The mempool contains the transactions submitted by users before they are either included into the chain or discarded after a timeout.

But I've seen that the tx pool currently has more than 100k tk in mem-pool, even though it fluctuates only up to 150k. https://etherscan.io/chart/pendingtx

That means, that the majority of tx as sitting there and taking up space. If I submit lots of tx with low gas fee (e.g. 0.1 gwei base), then as an attacker I can throttle the ethereum network by requiring them to transmit and store these tx in memory. Surely, this isn't good, right? It wouldn't cost an attacker much to resubmit the tx regularly. Wir say just 0.25 ETH they could submit lots of pending txs.

Or does the protocol allow any node operator to simply and arbitrarily reject txs from other nodes and from users if the gas is half as big as the 7 day minimum?