They (Digg) take care of the following things while the submissions are made,
(a) Submit things you think a lot of other people might be interested in.
(b) Link to the original source, not a site that's simply copying others' work.
Digg has recently made a few changes to the way Digg links to external sites
"These changes reduce the incentive to post spammy content (or link spam) to Digg, while still flowing ’search engine juice’ freely to quality content blog" - Digg the Blog
Digg have added rel=”nofollow” to any external link that they’re not sure they can vouch for. This includes all external links from comments, user profiles and story pages below a certain threshold of popularity.
The image shows the nofollow links marked in red for the upcoming section for Technology section.
I think this is pretty cool, as Digg isn’t adding nofollow to everything. Nofollow is only for the links about which Digg is not sure. Once a story looks real to them, they do remove the nofollow link. As such it would not be easy for the spammers to get the link juice for their websites.