1. When told about these accounts the activity on these accounts stopped.
2. The OP claims: "On a technical side: Digg can only ban accounts but cannot stop accounts from digging. So, if this was from some exterior group, digg would have only banned them as they cannot stop them from digging."
1 can easily be explained by recognizing that Digg may have decided to temporarily put a hold of the accounts once informed of the odd voting patterns.
2 is bogus. It's their site and they can do whatever they want. It should be easy to keep accounts live but stop their diggs from counting, for example.
It strikes me (see my other comment in this thread) that the accounts could have been created by people outside Digg manually (they are only roughly 100 accounts and they weren't created quickly) and then used for this purpose.
None of that implicates Digg.