Halfway through flowering the plants should pretty much be done growing tall. Various factors aside from genetics can contribute to elongated stems though, including light spectrum and temperature. HPS lamps tend to emit more red and orange light; contributing to long stems. Metal halide lamps emit more light in the blue spectrum and contribute to short, compact plants.
Temperatures in excess of 80 degrees can also cause plants to stretch as well as grow airy buds. This commonly happens when the tops of the plants are too close to the light (and the heat).
Excellent air circulation contributes to stouter, stronger stems.
Here is what happened to a plant I had to bend near the top. I basically bent the stem there gently and carefully: almost to the point of it breaking, then I used sisal twine to tie it up and keep it suspended (otherwise it would have broken). You can also use clips/Popsicle sticks and masking tap to secure it temporarily. I only had to do this in one spot, if you plan on doing it in multiple locations be extra careful. These will heal and stiffen up in a weeks time. Remember that phosphorous, potassium and calcium primarily promote strong stems. Silicon if available will also be taken up by the plant and used to strengthen cell walls.
Here are some pictures what resulted, (after harvest):
