You can't do it via HQL. If you think about it, its not really a valid thing to want to do. HQL works with mapped objects in a SessionFactory; what you are trying to do is remove the underlying table without removing the mapped object, which is asking for trouble.
You can do this via SQL, since Hibernate allows you to run SQL queries. But I'd think about why you need to do this first. And remember, Hibernate can create tables at start up time which is a more valid place to be performing DDL stuff.