Hi William,
AFAIK there is no way to do this natively with the SDK. Once created an attachment remains in the db.
Some objects allow lines to be removed using XML. But I've never tried this with attachments.
You can try the following:
Export the attachment to XM;.
Remove the attachment lines;
Reload the attachment;
Call Update on the attachment object.
Again this might not work, because some objects don't support this.
Good luck.
Best regards,
Pedro Magueija