I would sooner tell them what I thought and remove myself from the situation. I do that sort of thing for sport with soliciting calls, but you rely on their buy in to be affected by words. It makes it that much more easy for them to entrench themselves with their bullshit "well everyone's evil at the end of the day." Maybe you could do longer term pestering until they have to remove themselves from your presence (e.g. asking them to sign petitions, inviting them to protest themselves, putting them in anti-themselves chatrooms) but that's all pretty low % annoying behavior than it is effectual.

In my mind, and it's not the impenetrable fortress it's cracked up to be, the two ways to influence them are the marketplace of ideas (tm) and changing the societal superstructure. I pray for the day leftists can hold their ideas lazily and use the kind of bullshit arguments that uphold the status quo. "Well, capitalism means that you put people in concentration camps. Do you want to do that to your neighbor? They baked you cookies!"