I won't chastise you for not doing all your homework before hand because it's a bit late now, and lets face it that's not being very constructive.
First point I will address is the question "is there anything I can do to prevent it growing more than 8ft?". The answer here is that beyond genetics (which you can do nothing about) a snake's size is dependant on health and propper husbandry. If you are a lousy keeper and the snake is underfed, unhealthy, and living in substandard conditions it likely will not reach full potential adult size. This should NEVER be an option, so in short. If this sounds like a reasonable option, then you have no business owning this animal or any other. If you plan to care for your animal as you should (as I will assume you do), then no there is nothing you can do.
Now to the size questions. What you must first realize is that boas are sexually dimorphic, simply males and females will reach noticably different adult sizes. Males being smaller than females, they will likely not reach much more than 7 feet anyway, so here you're OK. Females however will certainly exceed 8 feet and may even reach or exceed 10 feet for this type of boa. But again in you're favour, boas are fairly slow growing animals and by the time she reaches that size you will likely be quite prepared, if not eager. As a mater of fact I would wager that most B.c.c. on this forum have not reached 9 feet yet.
I trust you will do more homework in the future, even if it's just asking on the forums for good and accurate info. And don't frett too much about the size of your animal right now, just focus on husbandry, as true B.c.c. are not as easy to keep properly as B.c.i. and for a new boa keeper (I assume this based on your questions) that is a considerable task.
Best wishes
Gary D.