Ask HN: how do you tell an investor you don't want them to invest?

I'm currently fundraising for my startup, and it's going way better than expected - well enough where I'm in the position for the first time of actually being able to choose my investors carefully.

Recently a friend of a friend overheard me explaining the product and our raise and asked for a pitch meeting. This person has been a serious point if contention with the friend involved, since I believe strongly this person lacks integrity and the friend disagrees. Anyway, integrity is our number one quality we look for in people we involve in the company and it has to be - we're in a sensitive space.

My question: how do I tell this person no without telling them that it's because I question their integrity?