You have two options:1. Completely block his emails from ever getting to you.2. Filter his emails into a special folder and use that as evidence when you take it to the next level.
To Block his emails:1. Sign in to Yahoo Mail.
2. Click the Settings drop-down menu.3. Select Options.4. Click Go.5. Click Blocked Addresses.6. Type in the email address you want to block in the "Add an address:" field.7. Click the Add this email address to the Blocked List button.
To Filter his emails into a Folder you specify:1. Sign in to Yahoo Mail.1. Click the Settings drop-down menu.2. Click on "help".3. Find the "Filters in Yahoo Mail" link and click on it.4. Read the instructions and watch the video on how to do it.
If its only harassment of your personal email you're all set. It it spills over into your experience in the group, then simply inform a moderator of the group and he or she can handle it, especially if you present him or her with evidence of the harassing emails.