Hi @sylviasphereo,

Do you have to have a Category?

Please read this doc: https://wpforo.com/docs/root/categories-and-forums/category-vs-forums/

I would like to test with a simple forum, can I archive the forum after a few days and keep all the activity, but close any more activity?

Sure, you can. You should create your forum and test it.Then if you don't need to let users to do any activity on your forum, just edit forums set No Access for all usergroups, except Admins.

I'd also suggest you read this post, here Robert has explained about forum configuration, and brought an example: 

https://wpforo.com/community/how-to-and-troubleshooting-2/create-a-forum-that-subscribers-are-unable-to-add-topics-too/#post-19309

Thank you Sofy - this is very helpful. I can see and understand all that you have said. 

I will try it all out, now.