Hmm, wouldn't they just be able to shuffle around some zaps over different npubs of which they are both sender and recipient? Seems very easy to circumvent.

i would argue that PoW (challenging your answer on another reply) requires an absolute penalty on spamming, but the penalty might be too low for a spammer while also being too high for new users.

While PoW on a single post is not so high, using it to send out thousands of replies would cost a lot of energy and therefore a lot of money.