Your understanding is true with 'Disposable Disk". They gets refreshed only while power-down initiated through VMware View. To remove System Disposable Disk from existing disk, first disable it on the pool configuration and then perform a Recompose (to a new image) or simple do a rebalance , so that VMs will get re-provisioned and the new settings will get applied.
On a side note, you can make use to "Space Efficient Sparse Disk" enabled for pool, and schedule the wipe and shrink on the disks, to save storage.
