I do think the spam hurts user experience. It's not so much the rewards themselves I'm concerned about it is the secondary effects. 1. Spam itself is distracting, and insufficiently controlled can be an outright denial of service. 2. Comments are very expensive operation on the blockchain (vastly higher than votes, transfers, etc.). Out of control spam comments will drive up RC costs and become an impediment to the value adding user experience we want.
We mostly lack the abilities that centralized platforms have to shut down (some, probably the vast majority of) spammers programmatically, or at all, by identifying bots and such (since it violates their ToS, and we don't have one), and to some extent to delete it or block it (since most UIs are on board with the "decentralized" ethos and don't want to be censors, and again, because it isn't violating a ToS), so we face a fundamentally bigger problem than they do.
The rewards incentive also changes the picture a lot (for the worse in terms of spam), as @onthewayout stated earlier.
Give authors free downvotes on comments on their posts?
Seems plausible.
Sounds like a decent idea and gives the author a little more sense of control over their own blog.