magento payment methods - enable for admin only

Two options:

1)
Override (using never change the original or add a /local/Mage/ override) the payment method (or just modify it if it's your own method), and add this:

protected $_canUseInternal              = true;
protected $_canUseCheckout              = false; 
protected $_canUseForMultishipping      = false;

2)
Create an observer on the frontend for "payment_method_is_active" event, and set to inactive the methods you don't want to show on the frontend:

   <config>
       <frontend>
           <events>
        <payment_method_is_active>
            <observers>
                       ... your observer here


 public function your_observer($event){

      $method = $event->getMethodInstance();
      $result = $event->getResult();

      if( $method should not be active in frontend ){

            $result->isAvailable = false;
      }

 }