When people talk about spam, they generally mean 2 things: high-volume cheap spam from lots of empty accounts, or a low-volume aggressive marketing from well funded non-empty accounts. The first one (it's DoS essentially) can only be properly prevented by LN paywall. The second one (let's just call it 'noise') - can be prevented by reputation filters, of which I believe TrustRank to be the proper one. 

We could use TrustRank even now to filter the first type of spam (I do at nostr.band), but then new people joining nostr would not become visible in a global feed without getting some friends to follow and interact with them (without gaining some reputation). LN is better because it filters out high-volume spammers without hurting the normal noobs.