To decline to respond or engage with a poster can be the best option in certain situations. For example, when confronted with clear violations/danger, you may block a poster and report them to the platform. You can also choose not to engage if the poster or message is in bad faith, or clearly produced by a bot or other bad actor. Finally, you may decline to engage to protect your own mental health.
Researchers find that responding to online trolls encourages them to continue their behavior, because their motive is to annoy and upset people. When you come across offensive and antisocial behavior online, you do not have to engage with the poster, and you may need to report them to the platform. Also, you may not want to engage with a poster or conversation where there does not appear to be a real willingness to discuss differences of opinion or reach agreement, especially when there is malice. By not engaging with such actors, you can conserve your energy and attention. In this case, you are also not rewarding bad behavior.