I cannot think of a single non-spam case where somebody would @-tweet the same link to a hundred people. That won't capture all spam, but it's a pretty easy low-pass filter.
My assertion is that after a certain point (which we are not far from), Twitter as a platform will have a problem making a distinction between "spam" and "legitimate content".
Where you can see the real problem is when there are trending hashtags that spammers get a hold of... spammers grab hold onto a hashtag and keep it artifically trending long past its relevance.