One more thing on this. IMO, you should NEVER permit this. The reason: this a waiver for the current plan and any other plan of the employer, regardless of whether it is in place now or not. Imagine a situation where a 25-year-old employee says, "I don't want to participate in this dumb plan." 10 years later, the employer amends the plan or starts another plan and the employee, now a 35-year-old with a spouse and 2 kids sees the value and wants to enter the plan. Don't you think he/she will then holler, "You never told me ...!!!!" Yes, they will likely lose in litigation (assuming that everything was in writing), but is this really the fight you want your clients to have.? In addition, as the others have said, if it's a small employer, it can cause you to fail coverage testing. Better to just amend the plan to exclude the person and then you can amend the plan to let them back in if they change their mind or if you need them for coverage.
Just sayin' ....
Ilene