Yes, but in the context of Freenet specifically, there's a lot of encryption and hashing involved in every transaction, which are computationally expensive, and there's no CGI equivalent at all (the whole network is basically static, append-only content addressed with the content's hash). So to create many messages you'd need to first generate the content, then encrypt and hash it, then insert it into the network starting at your node, then provide signature and checksum verification on every request. After some time, if the content is widely requested, some other nodes will cache it, but you can't really expect spam to propagate via web of trust; it's more probable that it will be added to spam filters which, themselves, will propagate. As everything is signed, you'd need to create a fresh node for each spam, but that means your new identity is not trusted by anyone - you'd need to convince at least some users that you're a user that won't abuse the system. Only to get your whole identity blacklisted once you do abuse it.

It wasn't perfect, and Freenet's usage was a huge PITA from the usability/ux perspective, but in a constrained environment like that this change in the protocol had much bigger impact than it would on the open web. Not sure how it turned out, but the trend until I left the community was very positive with regards to spam.