After looking through the archived help files for 'Block IP' it seems the easiest way was through the Guestbook tool. I found the box to enter the Ip address and tested it using another computer and it has not worked.
How do I ban a person from accessing my Message Forum?
1. Log into Bravenet and click "Message Forum"
2. Click on the Utilities button on the top left
3. Under Ban an IP Address, enter the IP you want to ban
4. Click on the Ban IP button
OR
1. Log into your account
2 ... return to your FAQ Manager page
5. In the Utilities area, click on "IP Banning"
6. Enter the IP in the Ban an IP address section, and click on "Add IP"