Learn to say "no". It's freaking easy. I do have a smartphone, and every time someone asks me to install something on it, or ask for my phone number for "accumulating bonuses", I say simple and dry "no" looking into their eyes. When there's a queue behind me, they'll just show loading icon on their face a couple of seconds, and then proceed, because of pressure. Otherwise, when there are no more people, they feel lik…
You can just as easily say "no thanks" and smile instead of being a creep who stares at people silently. Some checkout worker is not your enemy.
This does rather quickly establish the fact that, no, neither of us are giving our personal details to people who don't need them..... and asking for them is creepy.