All banning prohibits from writing new posts (replies, new topics, new polls) and private messages as well as registering new accounts.
Administrators can set a user's account to the status 'Banned'.
Administrators can define a list of IP addresses which should be considered banned. Partial definitions of IPs are considered as well, without the use of wildcards (i.e. if you define 192.168. as banned, the whole range of the local nets will be banned). This is a double-edged sword, always keep in mind that most people use the Internet with a dynamic IP address, so if you use this, you might punish the wrong person in the end.
People using matching e-mail addresses will be considered banned. This is relatively safe as long as you use either complete addresses or privately held domain names. For example, @spamboard.net will ban all addresses containing the former string, locking out only people who have an address on this domain. If, on the other hand, you enter @gmail.com, you'll lock out each and every GMail user (might not even be such a bad idea...).
This is only applied to posting, not registering, and only to guests trying to post. The guest's name is matched against the strings defined by the administrators (case insensitive). If one of the banned strings is found in the guest's name, the post will not be permitted. Of course, this won't stop the same guest from posting under a different name, but in some cases, it'll at least discourage annoying individuals. In the end, the only real solution to the annoying guests threat is taking away the posting rights of this group. Also, if your name ban definitions are too broad, you'll hit many other people again (consider entering 'e' as banned and watch the number of guest posts plummet...).