In the distributed implementation I would have a chain of trust from the people you follow, where if you report someone as a spammer, the server that represents people who follow you could look at your list and filter spammers based off this. You could also set the depth to traverse. There is a lot you could do with this.
I'm not saying my implementation is right, but someone needs to do it. Tweeting is a type of communication and needs to be distributed.
