What should I do about witnessing reprehensible acts and my inability to convince those close to me, what is my responsibility regarding advising strangers, and how should that be done?

Changing evil has stages: by hand, then by tongue, then by heart, which is the weakest of faith. Evil must be changed by every possible means, unless doing so results in a greater evil. If a person is unable to change it by hand, then by tongue; and if unable, then by heart, which is obligatory in all cases. Denouncing evil is a communal obligation (fard kifayah), and in some cases, it may become an individual obligation.