One nice phrase I've developed for saying "no" to strangers who ask me for spare change or to buy them food/coffee/whatever:
"I'm not feeling it right now, but feel free to ask me next time."
I really like this because:
I might genuinely be open to it next time! Especially if this is the first time with that person, I actually want them to know that.
It softens the blow at the chimp level: it's explicitly neither a total rejection, nor a rejection of them as a person.
It sends a signal against expecting a guaranteed payout pattern, especially if they've asked me before. That I'm happy to give one-off help, and am open to considering helping again, but not all the time.
It makes it clear that this is about how I am feeling, both in the nice way (maybe it has nothing to do with you, maybe I'm having a bad day or my own financial hardship, etc) and occasionally as a gentle reminder of harsher truths (you're making me feel like I am just a vending machine to you; just because I helped before, don't forget that random interruptions for a handout are unpleasant; etc).
I'm pretty big on helping people. Over the course of my life I've given more than average to random beggars/"bums"/etc. By default I assume they honestly need it. But it's important to protect your boundaries and not stretch yourself too thin, and this simple phrase is nice for that.
Just don't forget the old lesson from behaviorist psychology studies: random rewards are the strongest encouragement of behavior. So if you do this, you'll sometimes still need an iron-clad backstop against guilt, social pressure, and your altruism, because when you give permission to ask again, some people will - quite justifiably - take you at your word and ask again.
And that's part of why I like this phrase. It fits well into natural escalation if a person asks more often than I want, or if I catch the slightest wiff of them starting to see me as a free stuff dispenser:
"Sure, here's a few bucks" ->
"not feeling it, but feel free to ask another day" (breaking expectation that I will vend whenever pressed) ->
"not today either" (no longer inviting to ask again because I want to be asked less often) ->
"no. I'd like you to ask less often" (in case they need it made explicit) ->