- I'm not looking for a relationship.
- I have a lot going on in my life right now.
- I'll give you a call some time (not necessarily a rejection, but vague enough to go either way).
- I'm just trying to meet friends.
- I need to concentrate on school/kids/my business...
- I've been hurt.
- It's complicated.
- or when someone starts explaining why they'd be a bad person to date/love.
There's nothing wrong with not being attracted to someone. Say it up front. You don't have to tell them why. Leaving any doubt leaves you open for someone to keep trying. Not only that, but it's far more insulting when you assume that the person is too stupid to understand the situation. In fact, it's quite possible they're feeling the same way.
Saying something like, "I don't want to settle for less," is also a huge insult. You're saying that person is less than someone else. We're all good; we're just not all the right person for everyone. Imagine how boring this blog would be if that were the case.
We're all taking chances out there. Be the most honest you can be with regards to presenting yourself, and run with it. As my father used to say (and Sam Elliott in The Big Lebowski), "Some days you eat the bear, and some days the bear eats you."