- Lock the question as soon as it is answered. Sound a bit too drastic.
Locking is a moderator tool with good reason. For one the lock mechanism implemented by SE currently only allows completely locking a post, which would preclude any subsequent answers.
As it currently stands this is not feasible, especially since there are valid reasons for editing a post after an answer has been posted. Not all edits touch the code and not even all code edits are actually invalidating.
- Provide a radio button with a rollback reason, and post the reason as a comment.
IIUC the only thing that makes this different from what you're currently doing (assuming you're using something like AutoReviewComments SE) is that the comment is hard-baked into the UI. I don't think that's something we really want either, considering that rolling back an edit is a pretty unusual thing to happen to users on the internet. Allowing just the extra touch of "a human did this" can make a lot of difference in how that interaction is received.
Overall I can really understand that posting rollback comments over and over is annoying, but there are existing tools to make this less annoying and adding a "built-in" to the system seems to not provide a lot of benefit...