You can block those spammy users. Kind of a pain though... and the various clients would have to add a quick user blocking button.

You can set Twitter to only show @replies from people you follow, but that's obviously less than ideal.

I can see how this might be a problem.

But before we dig into this problem, let us pause for a moment and reflect on the age-old terrible plight of the tool developer. Twitter developed a tool that could not be spammed. So the users went busily to work, layering a spammable protocol on top of it. And now those users demand that someone figure out how to keep the spam out of their spammable protocol. In the meantime, Twitter gains a reputation as a system that's filled with spam.

Perhaps it is time for a corollary to Zawinski's Law of Software Envelopment: "Every social application evolves until it fills up with spam. Those that cannot be filled up with spam are replaced by those that can." Though I guess Doctorow proposed this theory years ago when he wrote All Complex Ecosystems Have Parasites:

http://craphound.com/complexecosystems.txt