Tom,

I don't know of a "proper" way to do this, maybe someone else does.

Here's a "sneaky" way I have used to do it:  

*       Create a devclass of type=FILE, pointing to a directory on disk.
*       Run a NEW DB snapshot using the new devclass..
*       Now your tape DBSNAPSHOT isn't the "last" one, you should be able to
remove it with DELETE VOLHIST to get your tape back.
*       Use OS commands to delete the disk file containing the last
DBSNAPSHOT to get your disk space back.

There will still be an entry in your VOLHIST for that last DBSNAPSHOT.
Don't know how to clean that up, but it's harmless.