Create a column called "validated" with "YES" as a value and upon deletion of others later on, use WHERE .... validated !='YES'
as an example.
DELETE FROM members WHERE validated !='YES'
AND passkey_valid <= DATE_SUB(NOW(), INTERVAL 1 DAY);
Your INSERT could resemble something like this:
$validated = "YES";
$query = "INSERT INTO members (validated,date)
VALUES ('$validated', NOW() )";