Suggestion:
Why don't you also store the rfid's onto the sd card, or file on the server, so you can remove single rfid cards.
Or you could just make the erase card only erase the next card, like:
-Tap erase card on reader
-wait for light to change
-Tap the card you want to erase (lets call it a) to reader
And then card A doesn't work.
Ive seen it done before with arduino (second method)