1. If the plug isn't too thick, just reach in through the saw's side slots with a screwdriver and pry the plug out, as shown. But if you have to drill through thick material, here are some tricks to use.
2. Drill partway through. If you have access to the other side of the stock, run the hole saw until its guide bit pokes through, then go in from the other side and complete the hole. That way the plug won't end up completely inside the hole saw and you can easily grab it and twist it out.