This really isn't a board problem.

Unfortunately when you have commonly used and widely known email address you start getting spam... it's inevitable. This is the reason why we have the moderator approval system in place... so that spam does not make it to the group as a whole. The MK64 group get's about 90% spam these days and in a few years we the MKDD group will as well... unless we are very careful or get very lucky. The best thing you can do to avoid getting spam is by not listing the actual email address for the group in any form on a webpage. Email harvester programs scour the web looking for email addresses and adds them to a giant database which the owner of the program sells to scammers.

In any case yahoogroups don't, to my knowledge, allow you to completely block an email account. Even if they did it's pointless because of the nature of SMTP. SMTP does not require sender authentication so you can pretend to be anyone you want when sending an email.