Because the web is stateless, people are effectively anonymous other than for that information that they provide about themselves.
The only fact that we know about a given poster to the Guestmap other than the information that they include in their post is the ip address that is currently allocated to their computer. Since many ip addresses are temporarily allocated from a pool belonging to the ISP this does not necessarily identify the computer from which the post was made sufficiently to ban that computer from making another post. The next time that computer is used it could be allocated a different ip address from the pool belonging to the ISP. If this is the case then banning the first IP address will have no effect, you would have to ban the entire range of IP addresses in the pool in order to block that one computer from access and that would have the effect of blocking every other computer connecting via that ISP from being able to post as well which could annoy a lot of legitimate visitors to your site.
On the remote chance that your problem poster has a permanent IP address allocated to them the Guestmap program has been amended to allow individual IP addresses (but not groups of addresses) to be banned from posting. A button appears under each entry when viewed via the Administrators screen allowing the IP address that was used to post a given entry to be added to the banned list.
Perhaps more effective at blocking someone from posting is to ban posts with a specific email address. Each email address belongs to a specific individual and so can block posts by that individual without impacting on any other visitor. Of course that person may have multiple email addresses and use a different one for each post requiring you to ban each in turn. Also the Guestmap does not require an email address to be entered and so this option cannot be used if an email address is not entered. A button appears under each entry that has an email address when viewed via the Administrators screen allowing that email address to be added to the banned list.
Note that you can ban both the IP address and email address associated with the same post if you wish. The actual post itself is not removed when you ban the poster from making another post. You can decide whether to also delete the post from the banned poster after you have banned them.
Should you decide to lift a ban that you have previously applied (perhaps because the IP address is being used by too many different visitors) then you can access the list of banned entries and select the entries to be deleted (and hence no longer banned) from there.
This article written by Stephen Chapman, Felgall Pty Ltd.