1. Go to Realtime >> Setup >> Communications >> [Clock Group] >> Identities. 
2. Click on the "Employee" header twice (to reverse sorting order by name).
3. Highlight the Unassigned IDs. Click on the first Unassigned entry, scroll down if necessary, hold down the SHIFT key, and click on the last entry. 
4. Click "Delete" on the bottom left.
5. Click "Save."