But if you really do know now, just tell them. And you can probably just tell them the reason, unless it's because it's a garbage place to work, in which case you should just tell them you don't think this is the right career choice for you long term and you're planning to look for something else.
I really really recommend that you avoid slamming doors shut before finding out if they’re the only ones that would be open for you. Don’t run to cut off opportunities. You really can’t know how the next six months will go for you.