With the software the race has either started or it has not - stopping the clock simply involves closing the clock screen out. If you open the clock screen again the program will continue to show the elapsed time between the race's start date/time and the current date/time, however it does not continue to run in the background when you close the clock screen out - think of it like the light in your refrigerator. So when the race is over, simply close the clock screen out.
If you practiced timing your event and need to reset the clock back to 00:00:00.000, press F2 to reset the clock.