Each time the form is used to subscribe a new email address the IP address is logged. If an IP address is adding lots of email address to your site you can set a time limit that needs to elapse between address submissions with this hook.

function lockout_period() {
    // number of seconds between data submits
    // default is 0 which disables this feature
    // 0 to 86399 are acceptable - 84399 is essentially a day
    return 86399;
}
add_filter( 's2_lockout', 'lockout_period' );