Here's a class I bodged together a few years ago to communicate with the
Stop Forum Spam API:
Code: <?php
class stop_forum_spam {
# Internal: Used to turn an XML response into a PHP array.
function process_response($response) {
$tags = array('appears', 'lastseen', 'frequency');
$parser = xml_parser_create();
xml_parser_set_option($parser, XML_OPTION_CASE_FOLDING, false);
xml_parse_into_struct($parser, $response, $vals, $index);
xml_parser_free($parser);
$result = array();
$type = 'unknown';
foreach ($vals as $val) {
if ($val['tag'] == 'type') {
$type = $val['value'];
} else {
if (in_array($val['tag'], $tags)) {
if (!isset($result[$type])) {
$result[$type] = array();
}
$result[$type][$val['tag']] = $val['value'];
}
}
}
return $result;
}
# Checks an IP address.
function check_ip($ip) { return stop_forum_spam::check(array('ip'=>$ip)); }
# Checks an email address.
function check_email($email) { return stop_forum_spam::check(array('email'=>$email)); }
# Checks a username.
function check_username($username) { return stop_forum_spam::check(array('username'=>$username)); }
# Checks multiple fields.
function check($fields) {
$url_components = array();
foreach ($fields as $k=>$v) {
$url_components[] = urlencode($k) . '=' . urlencode($v);
}
return stop_forum_spam::process_response(file_get_contents('http://www.stopforumspam.com/api?' . implode('&', $url_components)));
}
# Determines whether someone is a probable spammer or not.
function is_probable_spammer($fields) {
$is_passing_preprocessed_results = true;
foreach ($fields as $v) {
if (!is_array($v) || !isset($v['appears'])) {
$is_passing_preprocessed_results = false;
}
}
if (!$is_passing_preprocessed_results) $fields = stop_forum_spam::check($fields);
foreach ($fields as $v) {
if (is_array($v) && isset($v['appears']) && $v['appears'] == 'yes') return true;
}
return false;
}
}
?>
This is now used on the MaxCoderz forum. My inbox used to be flooded with activation requests from spammers attempting to use the forum; since I integrated this with the registration page I've only had one. 
To integrate into phpBB 3, open includes/ucp/ucp_register.php and look for
Code: if (!check_form_key('ucp_register'))
{
$error[] = $user->lang['FORM_INVALID'];
}
Add the following code underneath:
Code: ### STOP FORUM SPAM BEGIN ###
require_once("{$phpbb_root_path}include/stop_forum_spam.$phpEx");
if (stop_forum_spam::is_probable_spammer(array(
'username'=>$data['username'],
'email'=>$data['email'],
'ip'=>$_SERVER['REMOTE_ADDR'],
))) {
$error[] = 'You appear to be a spammer.';
}
### STOP FORUM SPAM END ###
That's it. Try to register an account for fanlynne for a demo. 
I'm flattered that you think we're running phpBB3 here, by the way. My main concern with that code is depending on an external API for verification and therefore being dependent on their up (or down)-edness. 
) now to go suspend that account