Some of the inactive users should be deleted from the Profiles database.
Solution with Websphere Commandline Interface
Unless you already know that this is done by configuring the Tivoli Directory Integrator (TDI), you will have to look it up in the Connections documentation. You need to create a text-file with all user distinguished names to delete, and then activate the TDI synchronization.
Solution with Connections Administration Toolkit (CAT)
Navigate to User Management – By User. Use the filter settings to create a list of users to delete.