You can ban problem members in a number of ways within your forum. Only administrators can ban members, features for moderators to ban/warn accounts will be introduced in future versions.
Ban Member Accounts
You can ban individual member accounts within the forum by associating the primary member group for any problem account to the "Banned Members" member group as shown below...
The "Banned Members" member group is a built-in member group that simply has an association to the "Banned Members" permission set. The "Banned Members" permission set has many of the post, editing & upload features disabled ensuring members within the "Banned Members" member group cannot post or take advantage of many of the features within the forum.
This will only restrict a specific member account based on the permissions defined within the "Banned Members" permission set. The problem member could register a new account using an alternative email-address to overcome the ban imposed on the member account. To prevent this for problem members you should also ban the members IP address as detailed below.
Ban IP addresses
This is the most reliable way to ensure problem members cannot post further within the your forums. If a users IP address is found within the Members & Groups » Banned IP Addresses list, attempts to register any additional forum accounts, login using any existing accounts or post any messages to the forum will fail. You can manage banned IP addresses from the Members & Groups » Banned IP Addresses page as shown below...