There should be a file in one of your directories containing the banned hosts/ips.
I have never worked or seen a DBX mud but if it is anything like any of the DIKU deriv's i have worked with, the file will be in /area (or possibly /doc, or /src, or similar) and will be called something self explanatory such as ban.txt or banlist or banned, etc.
Take a quick look through your directories for something like this. When you find it, edit it and simply delete any / all of the hosts/ips you no longer want to be banned, save it and there you go.
You should also have the code with your own wizard utilities that contains the do_ban command, that code should list/verify the location that the banned ip is sent to, so you could find the location there, if you can't find it by basic searching.
Hope that helps