Add this line
$wpdb->query("DELETE FROM `".$wpdb->prefix."adrotate_tracker` WHERE `ipaddress` ='unknown';");
Function now looks like:
/*-------------------------------------------------------------
Name: adrotate_clean_trackerdata
Purpose: Removes old trackerdata
Receive: -none-
Return: -none-
Since: 2.0
-------------------------------------------------------------*/
function adrotate_clean_trackerdata() {
global $wpdb;
$removeme = current_time('timestamp') - 86400;
$wpdb->query("DELETE FROM `".$wpdb->prefix."adrotate_tracker` WHERE `timer` < ".$removeme." AND `ipaddress` > 0;");
$wpdb->query("DELETE FROM `".$wpdb->prefix."adrotate_tracker` WHERE `ipaddress` ='unknown';");
}