There's a neat article over at the Social Science Research Network on the effect of stock-pumping spam emails on market price.
"For a stock that is touted at some point during our sample period, the probability of it being the most actively traded stock in our sample jumps from 4% on a day when there is no touting activity to 70% on a day when there is touting activity."
Long story short, the reason spam is still a significant problem is because it works.
