Earlier this week my anti-virus software was detecting viruses or sphishers in some of the ads that were trying to display on Yahoo sites including my e-mail and finance page. My anti-virus software blocked it.
Regarding the spammers that post messages on these boards containing links to 'stock picker' sites, I notice that on one of the Yahoo stock boards that I post on that after a while the posts containing links to stock sites disappear.
Perhaps Yahoo has software that detects them and then automatically deletes them.