You'd have to edita template and use a conditional I think, though I don't know what this would be as I haven't looked.
I think this would be pointless though as a user could simplyright click on the media (if a picture) and copy the image URL, so they could still share it. As for videos, they are already available on YouTube/Vimeo etc so no point prevent sharing these.
I'm not sure why you don't want other users sharing an owner's media? If it's for privacy reasons then make sure you set category viewing permissions appropriately and educate your users to set their album permissions correctly (you can set an appropriate default for albums in the ACP).