Cannabis is a pretty hardy plant, you can really cut them back to about 1/3 their size. This helps them in nature if deer come along and munch on them. Don't be afraid to chop of the top 2/3, the main lead and all. It may take them 2-3 weeks to grow some nice cuts back, but if you do this regularly it should take them atleast 8 months to get 6 feet tall.
This will also make them a lot bushier, so only a control for height, not width.
Another option Which i use is make each flowering plant the mother of the next clone .
Grow a topped plant to your desired height and then take off one of the 2 growing tops as a clone and flower the plant it came from this way you get the maximum veg time for a small plant . add infinitum