The obvious precommitment to make is to never carry cash.

I'm terribly sorry for my strong reaction but this whole post reeks of abuser attracting vulnerability so much that it's making me angry. It's not difficult to imagine beggars can sense you a mile away.

What the hell? People are robbing your time in the street and lying to your face to get your money too, and you are considering to inconvenience your own life to accommodate them? Just learn to tell a white lie like the rest of humanity, it doesn't even matter if you do it badly in a case like this. All you need is an attitude change, not a bag of tricks.

I'm going to appeal to your altruism. You're making lying for money profitable. When you give away your hard earned money it doesn't hurt just you or the potential charities.

I'm not sure what makes me so angry about this... it just seems that submissiveness seems to be a relatively common failure mode for otherwise smart people.

ETA: in Europe, begging is highly organized so you would likely be financing organized crime.

ETA2:

Then he told me that he had seen me around before - this is entirely plausible since I walk by there most days ... Is there some way to not look like an easy mark?

Yes, stop giving money to these people. Of course they recognize you, it's their job.