Go to privacy and settings at the top right of your page and click on Block Users, this is what CafeMom says for these kinds of issues:
Unfortunately, sometimes you might not like everyone at CafeMom. If you want to stop someone from commenting on your posts & photos, sending you messages, posting in your chatterbox, or sending you invitations, you can add them to your block list.
Be aware that users that you block will still be able to see all of your pages, they just can't add any comments. This setting also does not work within groups, only in the Journals/Photos/Profiles areas of the site.
at 1:27 AM on Jul. 16, 2008