I think, it's possible to do only directly in database. For example, by the following command:
delete from minibbtable_topics where forum_id=FORUM_ID;
delete from minibbtable_posts where forum_id=FORUM_ID;
But please note: number of postings per each users also needs to be recalculated. I recommend to run Update Script (convdb.php) after all and choose "second" option to re-calculate amount of topics and posts in static fields.
If you have large amount of posts, it's worth to recalculate only number of user postings (not also topic latest post time, as this script does by default). You may comment out the algorithm which states as /* Topic last post time */ and run this script calculating only user postings then.