After you have finished using your Docker persistent storage volumes you can remove them from the environment so you can clean them up and don't consume more storage that what is really needed. Here is what you have to do do clean those up.
List your volumes
docker volume ls
You will see your docker volumes stored in your /vsanDatastore/dockvols folder
Remove the volumes not needed
docker volume rm volumename
ex: docker volume rm MyVolume
This will remove the volume from the docker host and will also remove it from the VMware vSphere datastore.