Banning users and IP addresses is scheduled for part of the version 1.1 release, but that is a couple of months away at least.
In the meantime it can be implemented on the existing code base by modifying qa_limits_remaining() in qa-app-limits.php - simply return 0 from the function if you want to block the user identified by the $userid parameter or the IP address of the user making this request - you can get that from $_SERVER['REMOTE_ADDR'] - that's it!
