Being a very visible observer is often enough, if you can handle the potential heat if the (maybe) asshole realizes they look like a piece of shit and decides verbally berating you might turn you into the belligerent in people’s eyes. Just coughing loudly, and if they look around, then be visibly watching them can cause them to stop because they realize they’re under public scrutiny.

Ultimately, you can’t stop or be responsible if it gets worse out of the public eye, but you can help stop it then.