Try this phrase: “Ich nicht parlez español.”
So many, many things. Especially since so many talkers aren’t suffering so much as obviously having their regular fun.
Yes, not forcing yourself upon people you don’t know and who have not asked for your company making them suffer.
On the bus I saw a girl with a tee shirt that read, “let’s not ruin it by talking”. I thought it was hilarious. Especially since it was in Mexico, and few people could have read it.
Might try to get one. Then hope they can read.
So much of the above advice is to lie or to be really aggressive to get them to leave, or to explain yourself or make excuses in an attempt to be nice. This strikes me as a failure–a need to alter your behavior in order to do what you want, which is shop. Why not just continue your shopping? They can follow along and talk all they want, and you can respond however you feel like it in the moment, and they’ll either figure out your priorities or they won’t–but the result either way is you finishing your shopping and going home.
What makes you a target, I think, isn’t some obvious physical characteristic. It’s that when strangers test you out for conversation, you’re stopping to talk to them, and even sharing personal information. If they want a conversation with a stranger, it’s up to them to do what it takes to maintain it.
The same is true with anyone trying to make money from passersby on the street. They know the longer you talk to them the better their chance, so they’ll drag everything out as much as possible before getting to the point. If talking with you isn’t worth walking with you, they’ll finish the spiel faster, and they’ll get their “No” faster too.
Before the days of internet, people had an excuse for using random people as sounding boards.
But there are a kajillion message boards now where people are glad to provide a listening ear, from the comfort of their living rooms. Although you don’t get real time body language feedback, it’s superior in all other aspects. You can carefully select your words. You can do research and post helpful links to information. You can actually hear what is being said without being distracted by ambient noises or stress (which one would feel if they are hurrying to catch a bus but don’t want to be rude to a nice old man). You are much more likely to get a favorable response on the internet than you are chatting up random strangers in the real world.
There are also such things as pastors, priests, rabbis, social workers, therapists, and crisis counselors who are trained to provide guidance and listen. That’s what these people do for a living. It’s what they want to do. Strangers on the street have their own lives and are already tasked with being a listening ear for their friends and family. They deserve to be left alone.
People should be nice to one another, but they should also strive to not be pathetically needy.
This is important, too. Why are you stopping to talk to them? Just keep walking.
End it with a friendly wave and a “Well, see ya later!”
“I’m sorry, but I need to think about what I need to buy, but it’s been nice chatting with with you.”
If he keeps talking…
“I’m sorry, I can’t talk any more now.”
If he keeps talking, walk away, and come back to that section later.
What you have to not do is effectively ask him permission to leave. That may be more important than what you do say. Don’t say things like “Well, I really need to go, OK?”.
<Didn’t read every post.>
What I would probably do is lean forward and touch the person’s hand/arm (yes, even a complete stranger), and simultaneously, say with urgency, “I’m so sorry, but I really have to go. Good luck with all of that.” And then withdraw my hand, pull out my phone, turn and walk briskly away while putting the phone to my head. If phone not handy, I’d walk away looking at my watch.
Even if they read it, they won’t think it applie to them.
Another vote for earbuds combined with a vague and sweet smile, and walking away.
Whatever you do, don’t try speaking a foreign language. I know how to say, “I don’t understand” - and only that - in at least six languages, and I always pick the wrong one.
Okay, this one might work.
In an English-speaking country, I usually say “Sorry, I can’t understand what you’re saying” in Russian.
In most other countries, I say “Do you speak Swedish?” in Swedish. In both cases, that normally puts an end to the conversation.
Sometimes, just for fun, I’ll listen intently and nod, and then say “Tell me, do you ever hear voices?” If you’ve never tried this approach, I highly recommend it. The look on the other person’s face is always priceless!
“I must go now. My planet needs me”
Congratulations. You’ve just persuaded me that reading Edgar Rice doesn’t have to be a complete waste of time and brain cells, and can provide me with useful life-skills.
Now, see if you con do the same thing on behalf of Edward Elmer. 
“I’m sorry…I really oughta go mackdonna handheld shoehorn butterhorse.”
Just start it with, “I say, old [boy, chap, etc.]” That’s how the Brits do it, and it seems to work.
That’s very cool. Thanks for that link!
Take a cue from The Honest Toddler. Many of the techniques described herein can be directly applied to the situation in question.
Try reverse psychology by saying “let me let you go.”