Advice please... how to turn someone down after they ask for £200k

So I have another post asking if you'd give someone £200k ($260k) (after they wasted £3k which was for work + can't explain what happened to the money and you've given them a £26k annual salary job).

We've firmly decided NOT to give this friend £200k. He keeps calling us and we haven't returned his calls yet. He is a really sweet guy and we believe it's his girlfriend that put him up to asking (despite her saying she has rich parents - which we looked up and they aren't as well off as she claims) but they could easily ask her parents.

So yeah, both my SO and I are terrible at telling people no when we don't have a reason other than we don't feel comfortable with it. It just makes us feel/sound like dicks that can't be bothered to help. But after what happened with the previous money, we just don't feel good about it. And he asked very boldly and totally unashamed, he evidently didn't understand that gravity of what he was asking.

I literally need a script of what to say and how to say no. He keeps calling and I really should just explain we aren't going to give him £200k.

What would you say? How would you turn him down? What reason would you give?