I’d like to be able to lock a topic’s title without locking the topic itself. If there is such a feature, I can’t find it.
On my site, the topic titles should really be the purview of moderators, since they’re primary navigation elements. When users name them badly and moderators rename them, users should not be able to change them back.
There is no way to lock just a single post at the current time.
I would suggest talking to that user about their topic. What you suggest is at odds with the design principles of Discourse, where the user who created the topic indeed has a stake (though not “final word”) on what the title of their topic is.
That said users are awful at writing good titles in general so I sympathize.
How do I stop a user from having the final word? I couldn’t figure out how to do that without locking the whole topic.
Basically suspend their account. Unfortunately your options are to archive the whole topic, or “archive” the user. Hopefully it won’t come to that!
Maybe we could consider adding an “Edit reason” to title changes as well though? Gives the mod a chance to briefly explain why the change was made, and that they’d rather not have the user change it back.
For a long time I thought that most forum software disallow changing topic title by default. One by one I was corrected and though it made sense to “forbid” it at first, it sure does not anymore.