Seriously...
One REALLY good way to cut back on unwanted emails is to NOT go posting your actual email address on public forums...
I'm not trying to snide here, it's true. I am CERTAIN you have bought yourself on to at least one more list with this posting..
Spam exists, because Spam works. If I was a spammer (or even a spambot) I'd be looking to send you some emails regarding new spam blocking software... because now I know two things.. you have an email address, and you don;t QUITE know how to keep it secure.
My best advice at this point.. honestly.. would be to sign up for gmail account, and use that for dealing with your friends and family and serious communications and use your yahoo account to ... yup ... collect all the crap you have no intention of reading.
Yahoo, Gmail, Hotmail and all of the other guys out there are doing there best to filter this stuff, but the bad guys will ALWAYS be one step ahead.. that's life on the intertubes...