Thing is, she might check your profile from time to time to see if you have accepted or not. And if you don't do anything, she will still see the request as "pending" and may start bothering you about it.
It will be on *her* profile that the friend request is pending- if you press 'ignore' it will disappear, and if she's got heaps of friends then you will disappear into the ether unless she is looking for you specifically, or you post something that comes up on her front page.
If you hit 'ignore and block' then you will effectively become invisible to her on facebook- even if you have mutual friends, it won't publish anything about you (even if a friend of yours comments on a photo or post of yours) on her page.
I've done the 'ignore and block' on a few people who associate with friends of mine, and have heard through the grapevine (from a mutual acquaintance) that myself and a number of other people, according to this "clique", are I quote 'So freaking LAME they don't even HAVE facebook!"
I just want to go up to them and say in a very un-eHell approved manner 'well, yes I do have facebook, but I'd rather use it to communicate with FRIENDS than J3RKS like you!'