People shouldn't consider "spam" as being "buy cheap ****** now" type posts. Not every piece of content you write needs to be submitted to Digg, which I think is a key mistake a lot of people are making.
On high traffic / high content sites, which forums are a great example of, there is a very high percentage of content that isn't Digg worthy that may get submitted by a user or two just because they think it is. What is valuable to you, me or the other guy, isn't necessarily valuable to the "masses".
On forums, is a Digg button on every post really necessary? Keep in mind that the model of Digg asks for users to submit content on an individual basis - expanding that to EVERY page of your site is taking it far and is likely to get you banned.
Try submitting every blog post you make to Threadwatch, or posting it on your SEO forum of choice and see how long it takes you to get banned.
While it might not seem fair that just because your users are submitting your content you get banned, people need to respect the fact that Digg most likely recieves a LOT of dodgy self promoting submissions and they need to draw the line somewhere. I don't care how good a writer you think you are - every page of your content does NOT deserve to be Dugg!
As with any online marketing fad there has been out there, people are taking the piss and trying to get something for free when in reality they really don't deserve it. Inevitably, this means that the rules tighten making life harder for the rest of us.
Loads of spam submissions (content that shouldn't be submitted) + the rising popularity of Digg buttons on site wide content + site owners submitting ALL of their own content as it's published + fake users + top users being paid to Digg + legitimate users submitting content en masse = rules need to change or Digg would just become PRweb 2.0.
Social marketing is not like SEO - you can't simply apply the same concept or techniques to each page of your site and expect similar results. You need to choose your points of "attack" more carefully or you get burned with crap like this.