The fastest way to deny based on RBL is to add it to whatever ACL you specify in acl_smtp_rcpt

However, you MUST put the declaration AFTER any relay allow definitions. ACLs are based on first-match which means they run in order and stop when they hit a match. Implicit allow.

Here is my ACL declared as acl_check_rcpt

acl_check_rcpt:
  accept  hosts = :
  deny    local_parts   = ^.*[@%!/|] : ^\\.
  accept  local_parts   = postmaster
          domains       = +local_domains
  require verify        = sender
  accept  domains       = +local_domains
          endpass
          message       = unknown user
          verify        = recipient
  accept  domains       = +relay_to_domains
          endpass
          message       = unrouteable address
          verify        = recipient
  accept  hosts         = +relay_from_hosts
  accept  authenticated = *
  deny    dnslists = zen.spamhaus.org
          message = Message rejected because $sender_fullhost is blacklisted at $dnslist_domain see $dnslist_text
  deny    message       = relay not permitted

The RBL definition is toward the bottom, after we allow everyone in that we want in. This lets people relay via SMTP-AUTH or explicit allow before checking the RBL. If they aren’t allowed via anything we allow, then we check the RBL and die with a nice message.

Adverts:
The Exim SMTP Mail Server
Exim: The Mail Transfer Agent
The Exim SMTP Mail Server: Official Guide for Release 4
A Practical Guide to Linux(R) Commands, Editors, and Shell Programming
Understanding the Linux Kernel
Ubuntu Hacks: Tips & Tools for Exploring, Using, and Tuning Linux (Hacks)
Linux Pocket Guide

WARNING!  As of this writing, this information pertains to the 2.x branch of phpBB.  I strongly recommend you upgrade to the latest phpBB.  As of April 28, 2009 that version is 3.0.4.  Please see http://phpbb.com

NOTICE: I have an updated phpBB patch for the 3.0.x version of phpBB.  Please search this blog for phpBB or look at the phpbb category for more tips and tricks.

In your active template file profile_add_body.tpl find the line:

<!-- Visual Confirmation -->

ABOVE that line, add:

<!-- BOT HACK -->
   <tr>
      <td class="row1"><span class="gen">Are you a robot? *</span></td>
      <td class="row2"><select class="post" name="imarobot">
      <option value="yes">yes</option>
      <option value="yes">of course</option>
      <option value="nope">Humans choose this one</option>
      </select> <span class="gen">Bot buster... choose the right one.</span></td>
   </tr>
<!-- BOT HACK -->

In your forum file includes/usercp_register.php find this section of code:

   else if ( $mode == 'register' )
   {
      if ( empty($username) || empty($new_password) || empty($password_confirm) || empty($email) )
      {
         $error = TRUE;
         $error_msg .= ( ( isset($error_msg) ) ? '<br />' : '' ) . $lang['Fields_empty'];
      }

AFTER that part, add this:

// BOT HACK
         if ( empty($HTTP_POST_VARS['imarobot']) )
         {
            die('NO BOTS');
         }
         else
         {
            if ($HTTP_POST_VARS['imarobot'] != 'nope')
            {
               die('NO BOTS');
            }
         }
// BOT HACK

And there you go.