Sony DVS-9000 User Manual page 217

To undo the saving of an effect
Immediately after saving an effect, hold down the [STORE STATS] button and
press the [UNDO] button to undo the save.
216
Saving Effects
This saves the current effect in the specified register, and turns off the
[STORE] button. The [RCALL] button and [STORE STATS] button both
light.