This is the fastest way. Copy existing list and delete old one. Or do it from the database directly.
Copy list does not copy the subs, it is meant to allow for quick duplication of the existing setup of that copied list (e.g. you can fix an import error quickly).Copy the list will bring the list "schema" (custom fields) + contacts so its of no use.
It seems to be the fastest.Copy existing list and delete old one.
This probably takes longer if you really want to delete all subs from a list.bulk action from source