Hi Ryan,
We're not currently capturing the ip address during forum posts. I generally don't recommend allowing anonymous posts in the forum. If the user is authenticated you can find his ip address under user management. ie look them up in the member list, click manage user then look on the location tab to see their ip addresses. You can also lock the account of authenticated users.
However you could find the ip address of anonymous users in the IIS web logs by looking for posts to /Forum/EditPost.aspx and correlating the post times.
mojoPortal does have a way to block ip addresses under Administration > Advanced Tools > Banned IP Addresses
However one of the problems with automated spam bots is that they can be using lots of ip addresses so it can be a constant battle trying to ban all of them. By requiring users to be authenticated in order to post it can be much easier to block them.
Requiring users to register on your site to post shouldn't be a big barrier to site participation by real users, especially if you enable them to login using an existing account via the Janrain social authentication service so that they don't have to remember any new passwords.
Hope that helps,
Joe