Things may have changed in the last few years as blogspam is still pretty lucrative and has continued to advance, but I remember reading a study a few years ago (found here:
http://www.plagiarismtoday.com/2007/04/09/why-wordpresscom-i...) that made the conclusion that by barring Wordpress.com bloggers from using AdSense and other advertising services, they significantly cut down on the amount of spam Blogger and other services had to deal with. Wordpress.com does have WordAds (
http://en.blog.wordpress.com/2011/11/29/wordads/).
Otherwise, if you don't qualify for WordAds, self-hosting may be your best option.