Advertisers attempt to poison online conversations.

When I see Twitter accounts spreading spam or following me (clues include "affiliate marketer" in the bio, or the use of the affiliate-oriented cli.gs URL shortener service) I mark the accounts as spam, which blocks them.

Unfortunately, I don't know if Twitter considers affiliate marketers to be spam -- the company's definition of spam doesn't explicitly say. But the more junk that appears in Twitter, or the more spam-related hassles people have to deal with, the less likely users will find it to be useful.