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/