If you reject a friend request, that member cannot send you a new request. If you rejected the request by mistake or change your mind later, you can unblock it.
How to unblock
First you have to open the member's contact entry. To do this, either go to the contact's profile and click "Edit contact" on the left, or click on the pencil icon next to the contact under My contacts.
If you have rejected a friend request, it will indicate at the bottom of the "Friendship" field that the request has been blocked ("Friendship rejected (request blocked)"). Simply click on the little arrow on the right, select "Remove block" and save the change. Once you’ve saved the change, you can send the member a friend request or wait until you receive another friend request.