One of the great uses that K2 (Joomla Component) claims that their system can do is act as a download manager. However, it does not have the ability to restrict downloads to only logged-in users. I needed a system where we could display the K2 item, with a preview and description, but not actually let them download the attachments. This was not achievable using just the included K2 parameters / settings, so I made this hack:
LOOK IN THIS FILE:
/com_k2/templates/templatename/item.php (line 364)
Download the attachment (Item.php K2 Template Modification Code) for the actual changes you need to make.
(note: stock, this will appear in your K2 component folder (/components/com_k2). You SHOULD be creating a template override (/templates/yourtemplate/html/) using the technique described here.