if the content has already been indexed by the SE I believe the robots.txt won't by of any use.
As a reminder, hiding the board from the navigation will only prevent the board from appearing from any menu or board list. It's content will still be searchable.
You should remove the content manually from the SE console.
Another option would be to make the content private so that the crawler won't be able to crawl the board (hence it should remove the URLs from the cache).
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