List<PermissionSetAssignment> psAssignmentsToRemove = [SELECT Id 
                                                       FROM PermissionSetAssignment 
                                                       WHERE PermissionSetGroup.DeveloperName = 'CEC_JTAC_Engineer'
                                                       AND Assignee.IsActive = FALSE];
System.debug('Permission Set Assignment #: ' + psAssignmentsToRemove.size()); // Confirm # of users it will be removed from
Integer errorCounter = 0;
Database.DeleteResult[] deletedPSAssignments = Database.delete(psAssignmentsToRemove, false);
for (Database.DeleteResult result : deletedPSAssignments) {
    if (!result.isSuccess()) {
        errorCounter++;
    }
}
System.debug('Assignments not deleted: ' + errorCounter);