This is the solution I use in classifiedsbarcelona.com to prevent Spam from India.

Usage:

Edit your commmon.php find the function isInSpamList

And replace the whole function with this code (there’s more than 1 function be aware)

////////////////////////////////////////////////////////////
function get_tag($tag,$xml){
	preg_match_all('/<'.$tag.'>(.*)<\/'.$tag.'>$/imU',$xml,$match);
	return $match[1];
}
 
function valid_ip($ip){
	return ( ! preg_match( "/^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$/", $ip)) ? FALSE : TRUE;
}
 
function geoIP($stringIp=-1){
 
    if (!valid_ip($stringIp)) $stringIp = $_SERVER['REMOTE_ADDR'];
 
    if ($_COOKIE['geoip']){
        $geoip=unserialize($_COOKIE['geoip']);        
        if ($geoip['ip']==$stringIp) return $geoip;//only return if IP is the same if not continue
	}
 
	$url='http://api.hostip.info/?ip='.$stringIp;// Making an API call to Hostip:
	$xml = file_get_contents($url);//echo $url;
 
	$city = get_tag('gml:name',$xml);
	$city = strtolower ($city[1]);
 
	$countryName = get_tag('countryName',$xml);
	$countryName = strtolower ($countryName[0]);
 
	$countryAbbrev = get_tag('countryAbbrev',$xml);
	$countryAbbrev = strtolower ($countryAbbrev[0]);
 
	$geoip['ip']=$stringIp;
	$geoip['city']=$city;
	$geoip['country']=$countryName;
	$geoip['countryAb']=$countryAbbrev;
 
	setcookie('geoip',serialize($geoip), time()+60*60*24*15);// Setting a cookie with the data, which is set to expire in half month:
	return $geoip;
}
 
function isInSpamList($ip){//return is was taged as spam (in /manage is where we tag)
	global $ocdb;
	$res=$ocdb->getValue("SELECT idPost FROM ".TABLE_PREFIX."posts p where isAvailable=2  and ip='$ip' LIMIT 1","none");
	if ($res==false){
	    $geoip=geoIP();
	    if ($geoip['country']=='india' || $geoip['country']=='(unknown country?)')  return true;//ohohoho SPAMMER! from india
	    else return false;//nothing found
	}
	else  return true;//ohohoho SPAMMER!
 
}

This will make everything a bit slower but works fine to me.

Thinking about to add it in newer versions….with config in the admin of course.