The app subscribed me to a lot of podcasts I don't like, and it automatically downloads them
During the initial setup, the app recommended popular podcasts to you based on your chosen topics, and that there's an option to either add them as your subscriptions or remove them. The app also asked if you prefer to enable auto-download for your subscriptions. It's possible you overlooked these options.
Nonetheless, don't worry! You can always unsubscribe from those podcasts by accessing them from the Shows screen, and tapping on the round [Unsubscribe] button. Moreover, you can disable subscriptions auto-download in Settings > Downloads.