reject attendees by email address. place blocked email addresses in uploads/espresso/blocks.txt one email per line.

gistfile1.txt

function espresso_reject_attendee_by_email_address($attendee_data) {
	if(file_exists(EVENT_ESPRESSO_UPLOAD_DIR."blocks.txt")) {
		$blocks = file(EVENT_ESPRESSO_UPLOAD_DIR."blocks.txt",FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
		if(in_array($attendee_data['email'], $blocks)) {
			global $wpdb;
			$wpdb->delete(EVENTS_ATTENDEE_TABLE, array('id'=>$attendee_data['attendee_id']), array('%d'));
		}
	}
}
add_action('action_hook_espresso_save_attendee_data', 'espresso_reject_attendee_by_email_address');

// untill EE version 3.1.37 is released, you'll need to fix the bug on line 572 of includes/process-registration/add_attendees_to_db.php
// was: $ext_att_data_source['attendee_id'] = $attendee_id;
// change to: $ext_att_data_source['attendee_id'] = $ext_attendee_id;