Spam is addressed primarily at the blockchain level. Without question it has vastly improved since RCs and is hardly a huge problem at this point. I used to get dozens of spam comments on my posts (either my rare personal posts or burnpost), now I get a small handful. Even those often get downvoted (and hidden by UI) by others or by anti-spam efforts like mack-bot before I even see them. I think my experience is typical.

As far as abusive (in the sense of harassment), I think muting by the reader and potentially by the reader subscribing the curated mute lists is a sufficient remedy.

I don't think posters need or should have a much of a special right to curate/block/hide replies their own posts on a fundamentally shared platform. To the extent they're able to do this it should be less than absolute (and subject to community disagreement), for example by conveying some moderate bonus virtual SP to their downvotes.

Communities could change this dynamic, especially to the extent they shift to rewarding their own community-specific tokens and not global inflation. That becomes more of a "my house, my rules" situation and communities could set all sorts of rules on this (of course, more richness in this regard would be significantly harder to implement).

Spam has never been less of a problem on Steem than now, and with free downvotes upcoming, we may see that abusive (in the sense of harassment) may meet with a similar fate. At a minimum I would wait to see how free downvotes affect things before assessing anything.