There are 2 different sources for CFI expiration dates. There are standard user expiration dates, such as medicals and BFRs, and there are instructor-specific dates like an instructor certificate expiration date.
Standard user expiration dates
If you want to prevent an instructor from being scheduled that has an expired BFR, medical, or other user date, this option can be turned on in the Group Options page (Admin >> System Admin >> Group Options). This option is called 'Do not allow scheduling of instructor with expired dates'. If this option is turned on, the system will check the user-related expiration dates of an instructor, such as medical, BFR, etc. If those dates are expired, the system will not allow that instructor to be scheduled by a general user. Only a dispatcher will be able to schedule the instructor with confirmation of the warning.
Instructor specific expiration dates
Other dates that are not relevant to general users (e.g. instructor certificate expiration) can be setup up as Scheduled Maintenance items. If these items are marked as mandatory and are overdue, the instructor will not be able to be scheduled without a dispatcher override.