I agree with Chuck from Cincinnati. Sounds simple to me... I also wish we could DNF runners who play music via portable speakers during the race.
In response to:
Or you can use the non-technology method if they miss the cut off ? The official at the cut off, use a sharpie to write DNF on the bib when instructed they missed the cut-off.
I keep my bib numbers, and one of them has DNF, that I wrote on it.