It might be more convenient to clear the ‘private’ volume of the disposable template (e.g. default-dvm), resetting it to a pristine state containing no data. This function is not exposed in the tooling but it’s straightforward to invoke:
$ qvm-shutdown --wait default-dvm
$ qubesd-query --empty --fail dom0 admin.vm.volume.Clear default-dvm private
Or even simpler, revert the volume to its previous state - if you’re sure that you just dirtied it up the last time the disposable template was running, and that you haven’t started it since:
$ qvm-shutdown --wait default-dvm
$ qvm-volume revert default-dvm:private