[GenSec]

I like hellban idea, but there is another way.
If you created 666 usergroup you find in admin/ functions.php

PHP Code:

    if (!$ignore[$post[userid]]) {
        eval("\$retval = \"".gettemplate("postbit")."\";");
    } else {
        eval("\$retval = \"".gettemplate("postbit_ignore")."\";");
    } 


add below

PHP Code:

    if ($post[usergroupid]==666 AND $bbuserinfo[usergroupid]!=666) {
           eval("\$retval = \"".gettemplate("postbit_ignore")."\";");
    } 


and all your hellbanned user are on common ignore list.

Or change postbit_ignore template for more suited.