Whenever I say to an acquaitence that asks me to go somewhere...and i do not?

want to, I just have an excuse handy...Such as.."sorry, but I have other plans today."...and the nosy person says.."well, what are your others plans? I just say.."just other plans"...then say I've gotta go...and hang up the phone. This person is a pain...has no boundaries and asks way too personal questions.But that is her. What do you say to people like her??? (They are annoying)...geez!! Gets on my nerves.