Two or three dates its fine. But if she likes you, or you've done anything with her and her girl parts, you owe her an explanation. Even a crappy one.
I'd argue just cutting off and ignoring is the cowards way out, were I to do it, I wouldn't be able to look myself in the mirror.
Tip: I recommend NOT telling her she's not 'worth pursuing'. Make it about you (since it is) and go for something like 'I'm just not into it', 'I found someone else' even if that someone else is your xbox. Whatever. Man up and talk to her.