I agree with seeking out middle aged women, sitting in the aisle, ignoring, speaking up loudly, when needed, etc. Always employ those strategies first.

That said, a lot of people (guys, younger women) just don’t want to involve themselves and I’ve found a few phrases to deflect/be helpful - only after all other strategies have failed.

If complementing your smile/eyes: “thank you that’s so sweet, sir (use the most formal polite address you can - you want them to live up to it) that made my day, sir. My mom also loves it”.

Guys who do this want to see if you’ll engage with what they think is an above board complement - flatter their ego by responding, by using Sir they get that you are not going to engage and by bringing your mother into it you are signaling that there is zero chance this approach will work. If you don’t engage they get mad because they tried what they thought was a respectful approach.

If complementing another body part or saying something sexual in English: do not engage. RBF and fight your way out if needed. No exceptions.

If saying something sexual in another language with their friends around: learn how to say “do you speak to your mother/sister/daughter with that same mouth” in a variety of languages and then shake your head. Get up and change seats to get further away - leave as soon as safe. When I lived in Italy this was definitely incredibly effective.