Yahoo Email Address is being spoofed to send spam...

There're soooo many bots that searches the internet for email addresses. If your is being displayed somehwere, such as in the whois database, then you're good to recieve spams. It's very simple to have your email spoofed. Yahoo doesnt verify the validity of your account either (but it's irrelevant in this case). If they dont have access to your email account, then dont worry about it. The above mentioned cases where your friends account got closed is probably due to a trojan in their computer more than external spammers.