You can hide users in the Global Directory if you don't want them to be visible.
When you hide a person from the global directory, other users can’t invite them to new projects. However, they’ll still be able to access the projects they're already on for searching and reporting purposes. While users in your organization will be able to see their name in the Global Directory, users outside your organization won't.
Note that users can control their visibility in the Global Directory themselves. You can check a user’s current visibility status via Setup/My Organization/User Accounts List.
Log into Aconex and select Setup from the Module Menu.
Select User Accounts List under My Organization.
If you’ve made the change in error, you can unhide the person by clicking the Show in Global button.