I work on the Trust & Safety team at Twitter and hate spam as much as everyone else. As with any large system the solution is never as simple as "implement this thing, problem solved." As Marco points out, what is or is not spam is a balancing act. Our head of Trust & Safety, Del Harvey, gave an interview to the Guardian earlier this year about this balance:
http://www.guardian.co.uk/technology/2011/apr/07/twitter-int...
If you'd like to help us out we have several open positions on the team: Anti-Spam: https://twitter.com/job.html?jvi=oBPbVfwg,Job Tools: https://twitter.com/job.html?jvi=oSbdVfwV,Job Front End: https://twitter.com/job.html?jvi=owPbVfwb,Job