Is there any clever way to remove certain subforum and all its topics from google index?

I cannot do it with single url from google search console as the topics URL do not carry any relation to the subforum they belong to. 

Have a look at the Meta Tags section here

 

Add the sub-forum URL and add tags for robots and type in noindex,nofollow. That seems to work for me.

Thanks, looks interesting. But if I add the noindex to the subform URL, would that be enough? These topics might be linked directly from somewhere else and once the bot is able to crawl the subforum through such backdoor, I fear it will index all of them. 

I cannot think of a way to easily and globally block access to Googlebot for all topics within a subforum, unless you also block guests via the general permission system. You would have to do this individually (either through robots.txt or by adding the aforementioned meta tag).