• "We hired an employee, who told us they did not want benefits, but after 90 days, they want to be on the plan."
• 'We hired an employee, who told us they only needed a single plan, but once they become eligible, they now need a family plan."

How do you prevent this?  

The simple answer is you can not prevent it .    

But, one possible option is to look at your employer contribution formula for the employer sponsored group health insurance plan.   Similar to how your pension plan has a vesting schedule, you can have a sliding scale for the group health insurance plan based on years of service. This is one way, you limit the exposure when either of the above scenarios occur. This also rewards employees who have been with the company longer.    

Make sure you have your employer contribution in writing and that you treat everyone equally.