Is it possible to restrict Webaccess to prevent it from searching usernames in other domains in our system?

We belong to a very large Groupwise system with each school managing their own domain, but all of us sharing a huge system address book. Other schools periodically create users with the same username as some of our users and then our users cannot login to Groupwise via Webaccess. There isn't anything we can do about other schools, except to call them and ask them to rename their user. This is quite a hassle and in essence creates a denial of service for those users until it is resolved.

It would be quite helpful if we could somehow restrict Webaccess to only searching our own domain. We even tried using LDAP authentication, but that didn't help.