You can say 'Sorry, no' or just 'good luck'.
If you have any genuine sympathy that will probably come across to them.
In Portland, OR, which has a similar homeless problem to SF's, I learned to always include "sorry" when I turned them down, as when I did so, they almost never were rude to me in response. When I didn't say "sorry," I could never be sure what to expect.
As I read the article, I didn't really interpret it as he was actually giving out $100 a week, but rather saying that if you gave a dollar to everyone that asked, you could spend a lot of money.
We're also talking a specific week here, not at any given moment. I've never been to WWDC or SF during a big conference but if I was a homeless person looking for a place with a lot of people who is sure to have money, I'd look at the WWDC crowd.