As others have said, you have to purposely decline friend requests.

It's a hell of a lot easier to accidentally accept a friend request than it is to decline it.
And, in the event this was implemented, if you are completely absent minded enough to accidentally click "no", retrieve your friends list and then accidentally decline them a second time, then you could simply send them the friend request in return.

In before "What if they accidentally go through the arduous process of declining my friend requests on accident also".
(Sometimes I wonder how these people tie their shoes in the morning)