In essence you need to remove the macro "topics_action_link" from the appropriate macro - it depends which application you are using as to which that will be.
Hunt around after you click the "Edit this page" link towards the bottom of the forum page.
However, I am curious about why you would wish to prevent your users from quickly identifying which sub-forums contain the latest posts and generally to get some understanding of what are popular or long standing topics. It's a useful feature. If you search this forum you'll find that some actually want to make that the default view on their forum.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.