This is pretty easy to do!
Just add the following, to which ever method you require:
appcodecoreMagePaymentModelMethodCheckmo.php
protected $_canUseCheckout = false;
protected $_canUseForMultishipping = false;
Underneath:
protected $_code = 'checkmo';
See original post on: http://www.magentocommerce.com/boards/viewthread/79242/