Well, you can delete a thread, but this is kind of risky operation, because there is no such operation in the UI. And this was made on purpose. Because one of the main ideas in AsmBB is to limit the power of the administrators and moderators and give the visitors some more freedom to self-organize.
Of course the administrator can do everything with the database, but my intention was to make some things not so easy.
So, you can delete a thread and all it's posts only from the SQLite console:
delete from threads where id = %THREADID%;
delete from threads where slug = "%VERY_BAD_THREAD_SLUG%";
But, well, deleting from the database is kind of risky operation. Are you ready to delete all your database by typing one wrong word?