As indicated before, I do understand perfectly well how the forum accesses system works. Currently if "Can post reply" is disabled then nobody can reply, not even the topic starter. If it is enabled then everybody can reply.

I requested a new permission be added, "Can post reply to own topic" which is a similar permission to what XF/VB has (see attachment). The net effect of enabling the mooted permission is that only the topic starter will be able to reply to their own topic but nobody else will be able to. 

If the above is intended behaviour in the "Can post reply" permission but is currently broken and will be fixed in a future update great; if not please consider the feature request.

I just want to say that feature you need should be configured through Forum Accesses. So you should ask for a new permission in Forum Accesses. Reading your reply I supposed that you are searching for other solutions you don't want use the Forum Access. There is no other ways to do this configuration. The correct way is adding a new permission you've mentioned above in Forum Accesses. And as you know there is no such permission yet, so there is nor way to do this. I've already let our devs know about this feature. This is added in our to-do list, but I'm really sorry it'll not be included in next two releases. Probably we'll add it by end of next month.