This change was noted in the recent Arc 3.1 release discussion. Based on that comment I think the threshold is now 200 karma.<p><a href="http://arclanguage.org/item?id=10254" rel="nofollow">http://arclanguage.org/item?id=10254</a>
I can still downvote, so presumably the threshold has been moved up a bit.<p>Just write a few more insightful comments or submit some good stories and you'll probably be able to downvote again.
Downvoting is pointless. It encourages mass retribution against unpopular but valid topics.<p>As someone else said, if a topic is broken -- spam, stupid or otherwise useless -- it should be flagged appropriately. My two cents, or probably about six cents worth 1 cent in 1987 values.