The best way to handle it would be as a feature request. I can see the need for it and it should be implemented in the uStore back office on the property/customization setup configuration.
As a work-around today:
* If the product is using the legacy multi-page mode instead of the newer NG mode, then you could probably use the "add javascript to storefront" feature to automatically clear the value.
* If it is using NG, that will not work. In this case, the only thing I can think of that might help in the short term is to create a custom input dial:
Input Control Development Guide · XMPieLab/uStore-NG Wiki - However, I don't think the dial gets information from uStore about whether the dial is being loaded for the first time, or for editing an item that has already had the value set? So, you may end up clearing the value every time the dial loads which might not be desirable. Also IMHO having to create a custom dial just for this seems like overkill to me.