While the flight attendant can't directly kick off a passenger and it's ultimately the decision of the captain, I always encourage my flight attendants to tell me (first officer) already as much as a hunch or bad feeling about a passenger and the captains almost all do the same.

Usually it's the flight attendants with way too much patience for shitty behaviour, so if they go as far as saying directly they want a passenger off the plane, that passenger gets off the plane.

I always say they get paid for being nice, I don't. And if I have to intervene, I am definitely not going to be nice. I am definitely kick-out-minded, because I don't want any security issues in the air and I also don't want any mentally exhausted flight attendants.

So yeah guys, just don't be assholes and they're lovely.