No sales ads please

I'm a little disturbed this morning, I went on my blog and saw that there was a comment so I went to read it thinking one of my friends made a comment about my site and well I was wrong.
It was a sales ad for Nike Shoxs. How can they do this? I'm not sure how to respond to it and I don't want to make my site private because I want all my friends to read my blog.

I guess if anyone has any advice please let me know.