"instead of downvoting"
Disagree. Maybe in *addition* to downvoting, but the key thing is getting them off of the front page because then there's no motivation to keep posting spam.
We had a similar problem on http://www.rubyflow.com/ - a similar Ruby community site. Our solution for a long time was to only include items from "approved" users in the RSS feed. This worked very well but then people got annoyed they weren't added to the approved list and we didn't have enough mods to catch up with it.
So now we have the RSS feed on a 3 hour delay from the live site which means spammy posts are often reported or moderated away before they hit the RSS feed. It's not absolutely ideal but works given the circumstances. To create such a delay involves only the most basic of tweaks to the query that fetches the items for the RSS feed.
Now imagine someone malicious drawing a pixel perfect Chrome browser that someone thinks is a normal browser, browses to their "bank" site, and enters credentials. Oops. I'm not even sure how you could protect against an attack like that..