Hey, Brenda:
My thought would be that since the participant did not pay taxes on the amount being refunded (so to speak) that you would use whatever dist. code applied and indicate that no taxes were paid in box 4. The only hole in that theory is that the partiicpant should not be penalized for the employer's mistake. I just looked over my distribuiton code list that I printed out from Relius before I left PA and I can't find anything else that would fit.
When I left PA, Susq did not take any deferrals out of my last pay check. It is my understanding, however, that this may change in the future. I sure felt cheated when I got that check (and I am still not eligible for the plan here). Good luck with your problem and take it easy!
Jesse
EDIT:
I was just telling my boss, Jeff, about your question and his answer was this:
It's not a distributable event, so there would be no 1099. The company would need to refund the participant out of the comapny funds (assuming that the errored $$ was already paid to the plan), paying the federal and/or state taxes at that time. Whatever money was paid to the plan in error would be credited towards future er contribuitons to the plan.
I fell like something similar happend to me while we were at PB/PA. This might not be the answer that you are looking for, but it might be your best choice. Jeff has been in the business for quite awhile (he's from Delaware) and I always feel comfortable with his answers (not like some other people we know). Good luck!