End dating an element entry via fast formula based on custom field on their assignment

We use a custom field marked “Yes” to identify union team members. When this field is set to “Yes,” the employee is no longer eligible for the 401(k) element, so the existing element entry should be end-dated. Can a Fast Formula on the element check the union flag before creating the element entry? If not, can a Fast Formula check the flag before the element calculates to prevent a deduction? In either scenario, can a report identify employees for whom the element was incorrectly created or the deduction was incorrectly processed?