[Andreas]

Cheepo solution:

Place this in phpinclude_start

PHP Code:

$doignore[uid] = array(ignoreid1, ignoreid2);

if (array_key_exists($bbuserinfo['userid'], $doignore)) {
  $bbuserinfo['ignorelist'] = implode(' ', array_merge(explode(' ', $bbuserinfo['ignorelist']), $doignore[$bbuserinfo['userid']]));
} 


Where
uid is the UserID of the user wher you want to add entries to his ignorelist
ignoreid1, ignoreid2 are the User IDs of the users you want him to ignore, add as many as you need separated by comma.
If you got several ppl where you want to add entries: Repeat the $doignore line as often as needed and change the IDs.