I will sometimes stop to leave a comment, usually something along the lines of “I’m not here to argue, but this isn’t what the Catholic Church teaches. The Catholic Church teaches . . … Even if you disagree with our theology/reasoning, I hope to at least make our side a little clearer.” Then I’d remind anyone who tried to argue that I wasn’t there to argue, and maybe add a little comment that says something like “if you want to learn what the Catholic Church really teaches, try a Catholic site, not an anti-catholic site that will misconstrue and misrepresent the information”.
Often I get replies of people wanting to argue, but there are some people who will come around and say something like, “ah, I do not agree with you, but now I understand the Catholic position better, and it is different from what I thought.” And that’s always nice.
You’re not always going to get positive results and you’re not always going to get negative results. But that’s expected from the Internet. The biggest thing is that you’re being charitable in your discussions, startig discussions and not arguments, and that you’re not being forceful about it.