http://phpsnips.com/544/Blacklist-Lookup#.VBG-zfmSySo This blacklist lookup function is a modification of this snippet. The difference is, that it returns if it suspects an ip is a spammer. If 50% or more of the lookups think your a spammer, the function returns true otherwise false.

blacklist_lookup.php

<?php 
function blacklist($ip){ 
    $listed = true; 
    $dnsbl_lookup = array( 
        "dnsbl-1.uceprotect.net", 
        "dnsbl-2.uceprotect.net", 
        "dnsbl-3.uceprotect.net", 
        "dnsbl.dronebl.org", 
        "dnsbl.sorbs.net", 
        "zen.spamhaus.org" 
    ); // Add your preferred list of DNSBL's 
    $lookups = count($dnsbl_lookup); 
    $total = 0; 
    if($ip){ 
        $reverse_ip = implode(".", array_reverse(explode(".", $ip))); 
        foreach($dnsbl_lookup as $host){ 
            if(checkdnsrr($reverse_ip.".".$host.".", "A")){ 
                $total++; 
            } 
        } 
    } 
    $percent = ($total / $lookups) * 100; 
    if($percent >= 50){ 
        return true; 
    }else{ 
        return false; 
    } 
} 

if(blacklist($_SERVER["REMOTE_ADDR"])){ 
    die("Your on the blacklist!"); 
} 
// Continue with page