If your organization has reached your limit of licensed web users, you will no longer be able to create additional web users. To get back under your maximum number of web users, you can either inactivate or delete web users from the EMS Desktop Client.
Web users can be deleted from the Configuration > Web > Web Users menu. Web users can be inactivated by hitting the "Edit" button on the Web Users screen, then setting the Status field to "Inactive".
The following message will display when opening the Desktop Client if the web user license has been exceeded: