You can permanently delete (purge) all of the histories associated with your account. Be aware that any that are in a shared state (link or published) need to be unshared before you can purge them.
If you have any histories listed under "Histories shared with you", those can also be unshared. (a small fraction of those history's content count toward your own quota).
- Checking for active vs deleted vs permanently deleted (purged) datasets and histories https://galaxyproject.org/learn/managing-datasets/#delete-vs-delete-permanently
Thanks! Jen, Galaxy team