I'm using packer for ova files creation.
I'm running it on a nested ESXi.
I couldn't find a way to delete the cached iso, which is stored by default at: /vmfs/volumes/datastore1/packer_cache/*.iso
(assuming that I'm using datastore1 as my datastore).
Use the vSphere (web) client. Browse to the database, select the file(s) and remove them. See VMware vSphere 5.1 Documentation Center for more information.