getList()
should be the answer. DOC here http://doc.silverstripe.org/framework/en/reference/modeladmin#results-customization
So something like:
public function getList() {
$list = parent::getList();
if($this->modelClass == 'Product') {
$list->exclude('ProductType', 'Type1');
}
return $list;
}
If you have more than 2 ProductType
you can use an array to exclude multiple values like
$list->exclude('ProductType', array('Type2', 'Type3'));