If you cut off the flowering heads you might get some side shoots but you could also pull them up and replant more broccoli for a fall harvest. It's your call.

Rodney

The small side shoots aren't worth it IMO. Pull those and start new plants for fall. For future reference, even in your zone, broc will usually have bolted or be close to it by mid-late June. It just gets too hot for them.

Dave

Summer side shoots aren't worth much, but the same plants can produce prodigious side shoots when fall comes - some as big as the original main head. I always pick varieties for their side shoot production and leave them in place for fall.

If all I ever got from broccoli were the single heads, I wouldn't bother growing them at all.