I think there may be a different approach: let spammers (aka bots) flourish and let users upvote the most helpful ones.
In short: 1. Create a second type of Twitter account for bots. 2. Give users the ability to opt out entirely of seeing bot replies. 3. Give users the ability to upvote helpful automated replies.
I wrote a blog post with this suggestion a while ago: http://www.readwritehack.com/how-twitter-can-win-back-develo...