This would be a very easy code for bb_plugins.php (preferably paste it before Captcha code):
/* Prevent certain IP range from registration */
if(($action=='registernew' or $action=='register') and "{$cen[0]}.{$cen[1]}"=='67.159' and $cen[2]>=0 and $cen[2]<=128) die('Your IP address is banned for registration');
/* --Prevent certain IP range from registration */
Here, $cen variable is generated in index.php each time when forum action executes, so we could merely use it in the above code or similar process. Look up for
/* Banned IPs/IDs stuff */
