Telling people they're doing something annoying / dangerous / etc is surprisingly effective. Ask the person smoking next to you to move downwind. Ask the neighbor next door to turn down the TV. Ask the tourist in public transport to move their suitcase so it doesn't block the door. Works every time, most of the time.

Doing so politely is effective.

But yeah, it's kind of shocking how often people are like "I hate this thing that person is doing" and I'm like "Have you asked them to stop?" and they haven't. Just... ask? Worse case scenario they say no and you're in the same spot you were.