By default, Discourse is not designed to allow bulk permanent deletion of topics, but we can easily accomplish this via the Rails console, with the relevant commands as follows:
./launcher enter app
rails c
Topic.with_deleted.where(deleted_at: ...1.seconds.ago).limit(1000).destroy_all
Post.find_by_sql("select * from posts where topic_id not in (select id from topics)").each { |p| p.destroy }
If you also want to permanently delete deleted replies to topics, you can use this command:
Post.with_deleted.where(deleted_at: ...1.seconds.ago).limit(1000).destroy_all