How do I restrict User Entries of excessively long Sessions

Access the Program Session settings (open a program, select the session tab, click on the ID of the session, and view the session Time Period settings in the Session Details tab, under Availability):

This is where you can add a session time limit to set a limit of hours and not allow a session more than that many hours. (Many users set this as 8 hours, but you can enter whatever duration you want.)

Usa a feature like this or use the session times to limit user entries to the times listed, then the user will be prevented from entering excessively long sessions. i.e. start limiting client check-ins and check-outs to being between the session start and end times.