Well, hard to know not knowing facts and circumstances, but presumably the employees are notified of the ACA at hire, or in the employee benefits manual, or upon plan eligibility, or SOMETHING like that, so perhaps the argument might e along the lines (stated more nicely, perhaps) of the following:
"We're an employer, not a babysitter. We tell them (whenever - at date of hire, etc.) and it is their responsibility thereafter. Any employee who doesn't want it can make a new election - either to stop deferrals, or change to a different deferral amount. Not going to waste time and taxpayer dollars doing needless notices that aren't legally required."