How do I prevent the scheduling of users with expired certifications?

From Dashboard, navigate to Location Settings.  The default setting is Location and it's the one you will be using to block scheduling of users with expired certifications.  Look for the Certifications Rules section in the lower middle of the page.  Click on the dropdown to make your choice.

When you use the dropdown menu, you will have only two choices.  You can receive an alert about upcoming expiring certifications, or you can preemptively prevent users with expired certifications from being scheduled.

Remember to click Save once you have made your selection. Staff members with expired certifications will not be allowed to be scheduled until the issue is resolved.

Warnings are then displayed across all scheduling pages in SchedulePop.  For example, here is how the warning is displayed in an individual user's shift card.