What can be done depends a lot on the nature of the disruptive behavior.

Someone having a mental episode or a drug addict is a very different situation from hyperaggressive teens or a domestic dispute or brazen would-be muggers or perverts.

Generally speaking, you don't want to physically interpose yourself when the problem is a group of teens who are acting out aggressively or when there's some kind of heated domestic situation. The aggression can very quickly get focused onto bystanders who are attempting to get involved in those cases.