I'm not familiar with CVSS, but as a sysadmin, I'd take your problem to be an integrity issue - that is, one part of the system is incorrectly able to affect another part of it. In your case, that's user B is able to delete user A's comments.
It's unlikely a sysadmin would be able to resolve this problem without some app changes, but one could imagine a similar problem on (say) a network drive on a work server. User A saves an important document, but user B deletes it (and it never makes it to nightly backups). This would be treated as an integrity issue and we'd find a way to separate the users such that user A can read/write to an area, but B can only read from that area. We wouldn't call it 'availability' because the network drive was working (as advertised) throughout.
This also brings on the question of "as advertised". My network share example has some implicit "terms of service" (I say implicit, because I'm not sure anyone writes them down), as does your web app. Whilst it's unlikely many web apps that allow any user to delete any other user's content would be considered terribly useful, you could argue that it's supposed to be like that, and that you'd need an extra layer of software to separate users content. This is somewhat arguing semantics, but it may help to understand (my perception, at least) of 'availability' versus 'integrity'.