Once I download ditgital photos from my scan card, how can I erase the
card and use it again?

Hello.

Either look for the Erase All, Format Card or Delete All option in
your camera's built-in menu or from the software that lists the
pictures in your camera's card, select all the pictures and press the
"Del" button on your keyboard.

have you tried deleting the scan cards from your camera? or usually
the other way to delete the data is from the software provided from
your camera vendor.

It might help if you tell us what kind of camera you are using, and
then some one who reads this will have the exact same software that
came with your camera. They will have figured it out for a quick, easy
answer.

Every digi cam has a menu option in the camera, choose delete from the menu.
In almost all cam there is an option to delete pictures one by one and
also a feature to delete all pictures in one go. Choose delete and
confirm yes to the alert asking for confirmation of yru action.

U can also delete pic with the software which came along with cam. 
choose the help option in the software or press F1. and find
information about deleting pics from the camera.
SHiNE//