Style Pack does not add or take away SEO for the forums. Maybe one day, but not currently.
The easiest way to block search engines is with the robots.txt file in your site root folder. Not all search engines will honor your settings there, but most of the big/common ones do.
https://moz.com/learn/seo/robotstxt
A more precise method is to create a custom function that blocks search engines based on their user-agent string. You will have to specify every user-agent string you want blocked, but those user-agents will be blocked consistently. You will have to create a WordPress-friendly function to redirect them to a specific page or display a message to them, and to make it apply exclusively to the forum.
https://blog.sucuri.net/2020/08/web-crawler-user-agent-blocking-techniques.html
A layered approach using both of those techniques above will be the most effective.
Alternatively, you could just set all of the forums to private, which will accomplish the blocking of search engine crawlers, but guests will have to login to see the forum too.
Hope that helps put you in the right direction!
