Once you have entered a race and received your starting bonus, you become one of the competitors. It is impossible to leave the race.
However, if you add no equipment to your boat and do not click the "GO" button, you do not enter the start area, and you will never start the race. Your boat will not be listed as a participat, and the race will not impact your VSR ranking.
After you click the "GO" button on the boat setup screen, you are committed to the race and your boat is in the port, ready to leave when the race begins. You can choose to not play this race, but if your boat does not cross the finish line, you will either earn zero point or lose points, based on your VSR ranking.