Well, I googled for details and it said larkspur often doesn't germinate at temperatures (presumably soil temps) over 65 degrees. In June your soil is likely warmer than that.
Do you have a spot where the soil remains in the shade but the light is bright and the larkspur can grow up into the sun?
I know that's not often found but it's how I got my delphinium to bloom late and longer so long as it was well staked.
I had a lot of success with the taller plants rooted in the shade but growing up into the sunlight. I don't think I'd cut back the growth to delay blooming (as in chrysanthemums)
or you might just get multiple shorter stalks with fewer blooms, or nothing.

Thanks I will give that a try! I will try sowing on the north side of the neighbor's 6 foot fence. The 2 feet closest to the fence should have cooler soil.

It all depends on what kind of weather we have in May and June. If our Spring is mild then they may bloom for a much longer period than the books say. I've seen them bloom well into July before - with me doing nothing. Usually the final period of the blooming is pretty ratty. So you might do all the tricks and succeed on extending the bloom but you might not be happy with the view. In my garden the leaves start to go downhill way before the flowers do.