This is why i like using GMail for all my e-mailing needs. Their blockers catch just about all the junk mail sent my way. If one happens to slip by just mark it as spam and google will automatically get it next time they try.
You could solve some of that by going into your yahoo mail options and mark your address as to not sell to third parties. That's how many of these things get addresses- they buy them. Others just mail a load of random addresses and hope to hit some that are actually working.
One way to get rid of spam is to use a "disposable" e-mail address. There are some sites that offer this sort of thing for cases like ordering something from a site. It will work for a certain length of time and then poof- gone. just another dead address on some spam sending list.