You could use a modObjectGetListProcessor, and then override the iterate() method and call object->remove() inside the loop.
You could also call $this->modx->runProcessor() on modObjectRemoveProcessor if you want to use all the permission checks, etc.
You could also do the permission check yourself (if it's needed) and, use getCollection() to get the objects, and then remove them:
<?php
/* I'm too tired to do the query ;) */
$objects = $modx->getCollection('modWhatever', $c);
foreach ($objects as $object) {
$object->remove();
}
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html