Sharepoint - clear a document metadata property

Can anyone tell me how you go about clearing a property of a document that is stored in a Sharepoint document library?

I have tried using a "Sharepoint documents" event, and updating the document's metadata field to a blank field, a space and not assigning anything at all, however I do not seem to be able to clear the value - it just stays as it was before.

If anyone has achieved this, and can let me know what I am doing wrong, I would be very grateful.

Thanks in advance!!