Over at math.SE (where I'm a moderator) we've been having problems with a user who isn't voting or accepting answers to questions. This seems completely against the spirit of the site, but I'm not sure to what extent it's my responsibility to make sure that users do this. What's the general opinion on this?
Yes this is against the spirit of the site, however we already display the "Mark of Cain" on near their avatar showing the accept rate.
Lack of voting is highlighted both on their profile and the new http://meta.stackoverflow.com/users?tab=voters page.
One of our most important/influential users on Stack Overflow almost never votes.
I think that as a moderator you should look at each problem holistically.
And so on...
For example: I would never contact Eric asking him to start voting, he does plenty to make our site awesome.
However, a serial asker that asks average to bad questions and never accepts may get the boot.