It may be inedible, but it's worth giving a leaf a taste to see if it is not too bitter. If it doesn't taste too bad, harvest the whole lot and eat some baby spinach salads.
There may be some spinach varieties which are more resistant to bolting, but it's probably a better idea to simply let nature do what it's going to do. Some years are warm, and some crops (e.g. spinach) don't like warm weather. A few years ago it was very cold and wet in June here and everybody had trouble with strawberries and tomatoes; there's not much you can do about the weather.
Something you can try is to locate your spinach (and other cool-weather crops) in a microcliimate where it stays cooler: perhaps where there's shade from the midday sun, a low area that cools off quickly at night, or near water that helps to moderate the temperature. You could also try putting some shade over the rows of spinach, e.g. a lath screen as shown in this book:
You can also hedge your bets: plant a variety of crops that are tolerant of different sorts of weather. For alternate greens, when you plant your spinach, start some lettuce (which still doesn't like heat, but is generally slower to bolt in my experience), and chard a couple of weeks later (which holds up really well to the summer heat).