There is a Delete offline stories button under Preferences > Offline Stories, but I found bugs in both that button and automatic cleanup. Cached article text was being left behind, and deleting stories wasn’t shrinking the database file.
I’ve fixed those for the next iOS update (v15, it’s huge and kinda amazing, huge performance improvements all over the app + add and discover sites + iphone duo support). Manual deletion will clear cached stories, text, and images and reclaim disk space. Automatic cleanup will respect your Stories to store limit, prioritizing unread stories and keeping recent read stories in the remaining space.