ioFTPD doesn't have a built-in command to add an IP to the ban list, only to view the banlist (SITE BANS LIST) and to remove a banned ip (SITE BANS REMOVE 10.0.0.1)
This means, if u want to "manage" a list of banned ip's, u will need an external script anyway. I don't see any reason why i should implement this in ioGui, if there already is
such a script.
Install that script, and edit
settings.ini
menu_activity = Ban ip %(clientip) | SITE ADDBAN %(clientip)