By $result = Mage::getModel('shipping/rate_result');
method I can create a new shipping rate. But How can I restrict other shipping methods in this action?
Grab All active shipping methods, loop through each one, and see if it matches your desired one
$activeCarriers = Mage::getModel('shipping/config')->getActiveCarriers();
foreach($activeCarriers as $code => $method) {
if($code == 'yourcode') {
$result = Mage::getModel('shipping/rate_result');
}
}