When deleting members, you must first Deassign Card from the member's account in order to be able to associate the card with another member. If you do not deassign the card prior to deleting the member, you will have to contact firstname.lastname@example.org and Kisi will deassign it for you.
To deassign a card, select the member from whom you wish to deassign the card. In the Cards window, you will see the card token that is assigned to that member. Click the three little dots to the right hand side and select Deassign.
You can also follow the steps in this video.