Been there, I don't know the proper way to fix it.
But here's a sneaky thing you can do if you have some free disk space:

Do another DBSNAPSHOT to DISK.
(You'll have to set up a FILE type DEVCLASS).
Then you will have 2 DBSNAPSHOTS, so you can delete the DBSNAPSHOT that is
on tape in your library.

Then use whatever operating system command to scratch the disk DBSNAPSHOT,
and free up your disk space.
You will be left with a bogus entry in your VOLHISTORY file, but at least
it's not taking up space...