Open the coupon and switch Is Active? off, then Save. Customers who try the code see "This coupon is not active", auto-apply stops, and the coupon is listed under Inactive. Switch it back on later to reactivate it. New coupons default to active.

There is no delete button for coupons — deactivating is the way to retire one, which also keeps its usage history (the Checkouts Used panel) intact.