I couldn't see any available filters for this, when I skimmed through the plugin source.
I wonder if it would work, if you replace line #1323 of the wp-polls.php
file:
if($check_voted == 0) {
with:
if( $check_voted == 0
|| ( is_array( $check_voted ) && 2 >= count( $check_voted ) ) ) {
to limit the number of votes per IP to 2.
Additionally:
Replace line #140 with:
if( !is_array($check_voted) && intval($check_voted) > 0
|| (is_array($check_voted) && sizeof($check_voted) > 1)
|| ($poll_active == 0 && $poll_close == 1)) {
But I don't recommend in general to modify plugin files, since it will be restored in the next plugin update.
Sidenote: The plugin is not using the recommended $wpdb->prepare()
when preparing the SQL for $wpdb->query()
.