Version 9.0 of Subscribe2 introduced an automated cleaning feature that deletes unconfirmed public subscribers from the database 28 days after the email address was entered. This hook allows the duration to be change or for this feature to be turned off.

function my_cleaner_interval() {
    // return the number of days before removal
    // returning 0 turns this feature off
    return 14;
}
add_filter( 's2_clean_interval', 'my_cleaner_interval' );